Display device and manufacturing method thereof

ABSTRACT

The display device includes first to third wirings, a first transistor, first to third conductive layers, and a first pixel electrode; the first wiring extends in a first direction and intersects with the second and the third wirings; the second and the third wirings each extend in a second direction intersecting with the first direction; a gate of the first transistor is electrically connected to the first wiring; one of a source and a drain of the first transistor is electrically connected to the second wiring through the first to the third conductive layers; the second conductive layer includes a region overlapping with the third wiring; the first conductive layer, the third conductive layer, and the first pixel electrode contain the same material; the first wiring and the second conductive layer contain the same material; the first wiring is supplied with a selection signal; and the second and the third wirings are supplied with different signals.

TECHNICAL FIELD

One embodiment of the present invention relates to a display device anda manufacturing method thereof

Note that one embodiment of the present invention is not limited to theabove technical field. Examples of the technical field of one embodimentof the present invention disclosed in this specification and the likeinclude a semiconductor device, a display device, a light-emittingdevice, a power storage device, a memory device, an electronic device, alighting device, an input device, an input/output device, a drivingmethod thereof, and a manufacturing method thereof.

Note that in this specification and the like, a semiconductor devicegenerally means a device that can function by utilizing semiconductorcharacteristics. A transistor, a semiconductor circuit, an arithmeticdevice, a memory device, and the like are embodiments of thesemiconductor device. In addition, an imaging device, an electro-opticaldevice, a power generation device (including a thin film solar cell, anorganic thin film solar cell, and the like), and an electronic appliancemay include a semiconductor device.

BACKGROUND ART

In recent years, a high-resolution display device has been demanded. Forexample, full high-definition (the number of pixels is 1920×1080) hasbeen in the mainstream of home-use television devices (also referred toas televisions or television receivers), while display devices havinghigh resolution such as 4K (the number of pixels is 3840×2160) or 8K(the number of pixels is 7680×4320) have been developed.

A liquid crystal display device is known as one of display devices. Atransmissive liquid crystal display device adjusts the amount of lightfrom a backlight to be transmitted and shows contrast to display animage by utilizing optical modulation action of a liquid crystal.

A thin film transistor whose channel formation region is formed using asemiconductor film that is formed over a substrate having an insulatingsurface is known as a kind of field-effect transistors. Patent Document1 discloses a technique in which amorphous silicon is used for asemiconductor film that is used in a channel formation region of a thinfilm transistor. For example, in the case of a liquid crystal displaydevice, a thin film transistor is used as a switching transistor in eachpixel.

PRIOR ART DOCUMENT Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2001-053283

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In the case of a display device such as a television device or a monitordevice, the higher the resolution is or the larger the screen size is,the more significant an increase in the load of a transistor or the likeincluded in the display device becomes. This might make operation at ahigh driving frequency difficult particularly in the case where thefield-effect mobility of the transistor is low.

An object of one embodiment of the present invention is to provide ahigh-resolution display device and a manufacturing method thereofAnother object is to provide a display device that is suitable forincreasing in size and a manufacturing method thereof Another object isto provide an inexpensive display device and a manufacturing methodthereof. Another object is to provide a display device with highproductivity and a manufacturing method thereof Another object is toprovide a highly reliable display device and a manufacturing methodthereof. Another object is to provide a display device using amorphoussilicon or the like and a manufacturing method thereof Another object isto provide a display device using a metal oxide or the like and amanufacturing method thereof. Another object is to provide a noveldisplay device and a manufacturing method thereof.

Note that the description of these objects does not disturb theexistence of other objects. Note that in one embodiment of the presentinvention, there is no need to achieve all the objects. Note thatobjects other than these can be derived from the description of thespecification, the drawings, the claims, and the like.

Means for Solving the Problems

One embodiment of the present invention is a display device including afirst wiring, a second wiring, a third wiring, a first transistor, afirst conductive layer, a second conductive layer, a third conductivelayer, and a first pixel electrode; in the display device, the firstwiring extends in a first direction and intersects with the secondwiring and the third wiring, the second wiring and the third wiring eachextend in a second direction intersecting with the first direction, agate of the first transistor is electrically connected to the firstwiring, one of a source and a drain of the first transistor iselectrically connected to the second wiring through the first conductivelayer, the second conductive layer, and the third conductive layer, thesecond conductive layer includes a region overlapping with the thirdwiring, the first conductive layer, the third conductive layer, and thefirst pixel electrode contain the same material, the first wiring andthe second conductive layer contain the same material, the first wiringis supplied with a selection signal, and the second wiring and the thirdwiring are supplied with different signals.

Alternatively, in the above embodiment, the second wiring and the thirdwiring may be electrically connected to a first source driver and asecond source driver.

Alternatively, in the above embodiment, a fourth wiring, a fifth wiring,a sixth wiring, a second transistor, a fourth conductive layer, a fifthconductive layer, a sixth conductive layer, and a second pixel electrodemay be included; the fourth wiring may extend in the first direction andintersect with the second wiring, the third wiring, the fifth wiring,and the sixth wiring, the fifth wiring and the sixth wiring may eachextend in the second direction intersecting with the first direction, agate of the second transistor may be electrically connected to thefourth wiring, one of a source and a drain of the second transistor maybe electrically connected to the fifth wiring through the fourthconductive layer, the fifth conductive layer, and the sixth conductivelayer, the fifth conductive layer may include a region overlapping withthe sixth wiring, the fourth conductive layer, the sixth conductivelayer, and the second pixel electrode may contain the same material, thefourth wiring and the fifth conductive layer may include the samematerial, the fourth wiring and the first wiring may be supplied withthe same selection signal, and the second wiring, the third wiring, thefifth wiring, and the sixth wiring may be supplied with differentsignals.

Alternatively, in the above embodiment, the fifth wiring and the sixthwiring may be electrically connected to a first source driver and asecond source driver.

Alternatively, in the above embodiment, the first transistor may includea first semiconductor layer, the second transistor may include a secondsemiconductor layer, and the first semiconductor layer and the secondsemiconductor layer may each include a portion positioned between thethird wiring and the sixth wiring.

Alternatively, in the above embodiment, the first semiconductor layerand the second semiconductor layer may each contain amorphous silicon.

Alternatively, in the above embodiment, the first semiconductor layerand the second semiconductor layer may each contain microcrystallinesilicon or polycrystalline silicon.

Alternatively, in the above embodiment, the first semiconductor layerand the second semiconductor layer may each contain a metal oxide.

Alternatively, in the above embodiment, the metal oxide may containindium, zinc, and M (M is aluminum, titanium, gallium, germanium,yttrium, zirconium, lanthanum, cerium, tin, neodymium, or hafnium).

Alternatively, one embodiment of the present invention is amanufacturing method of a display device; the manufacturing method of adisplay device includes a step of forming a gate line and a firstconductive layer; a step of forming a first insulating layer; a step offorming a semiconductor layer; a step of forming a first source line, asecond source line, and a second conductive layer and a third conductivelayer each including a region in contact with the semiconductor layer; astep of forming a second insulating layer; a step of forming, in thesecond insulating layer, a first opening portion reaching the secondconductive layer, a second opening portion reaching the third conductivelayer, and a third opening portion reaching the second source line, andforming, in the first insulating layer and the second insulating layer,a fourth opening portion and a fifth opening portion each reaching thefirst conductive layer with the first source line therebetween; and astep of forming a pixel electrode electrically connected to the secondconductive layer through the first opening portion, forming a fourthconductive layer electrically connected to the third conductive layerthrough the second opening portion and electrically connected to thefirst conductive layer through the fourth opening portion, and forming afifth conductive layer electrically connected to the second source linethrough the third opening portion and electrically connected to thefirst conductive layer through the fifth opening portion.

Effect of the Invention

According to one embodiment of the present invention, a high-resolutiondisplay device and a manufacturing method thereof can be provided.Alternatively, a display device that is suitable for increasing in sizeand a manufacturing method thereof can be provided. Alternatively, aninexpensive display device and a manufacturing method thereof can beprovided. Alternatively, a display device with high productivity and amanufacturing method thereof can be provided. Alternatively, a highlyreliable display device and a manufacturing method thereof can beprovided. Alternatively, a display device using amorphous silicon or thelike and a manufacturing method thereof can be provided. Alternatively,a display device using a metal oxide or the like and a manufacturingmethod thereof can be provided. Alternatively, a novel display deviceand a manufacturing method thereof can be provided.

Note that the description of these effects does not disturb theexistence of other effects. Note that one embodiment of the presentinvention does not necessarily have all these effects. Note that effectsother than these can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 A structure example of a display device.

FIG. 2 A structure example of a display device.

FIG. 3 A structure example of a display device.

FIG. 4 A structure example of a display device.

FIG. 5 A structure example of a display device.

FIG. 6 A structure example of a display device.

FIG. 7 A structure example of a display device.

FIG. 8 A structure example of a display device.

FIG. 9 A structure example of a display device.

FIG. 10 A structure example of a display device.

FIG. 11 A structure example of a display device.

FIG. 12 A structure example of a display device.

FIG. 13 A structure example of a display device.

FIG. 14 A structure example of a display device.

FIG. 15 A structure example of a display device.

FIG. 16 A structure example of a display device.

FIG. 17 An example of a manufacturing method of a display device.

FIG. 18 An example of a manufacturing method of a display device.

FIG. 19 An example of a manufacturing method of a display device.

FIG. 20 An example of a manufacturing method of a display device.

FIG. 21 An example of a manufacturing method of a display device.

FIG. 22 An example of a manufacturing method of a display device.

FIG. 23 Structure examples of a display device.

FIG. 24 Structure examples of a transistor.

FIG. 25 A structure example of a transistor.

FIG. 26 A structure example of a transistor.

FIG. 27 A structure example of a transistor.

FIG. 28 Structure examples of a transistor.

FIG. 29 A structure example of a transistor.

FIG. 30 Examples of a laser irradiation method and a lasercrystallization apparatus.

FIG. 31 Examples of a laser irradiation method.

FIG. 32 A structure example of a display panel.

FIG. 33 Structure examples of an electronic appliance.

FIG. 34 A block diagram showing a display module in Example 1 and acircuit diagram showing a pixel in Example 1.

FIG. 35 Top views showing a pixel layout in Example 1.

FIG. 36 Results of rough estimation of data writing time in Example 1.

FIG. 37 Results of rough estimation of data writing time in Example 1.

FIG. 38 A block diagram showing a display module in Example 1 and acircuit diagram showing a pixel in Example 1.

FIG. 39 Top views showing a pixel layout in Example 1.

FIG. 40 Results of rough estimation of data writing time in Example 1.

FIG. 41 Results of rough estimation of data writing time in Example 1.

FIG. 42 Results of rough estimation of data writing time in Example 1.

FIG. 43 Results of rough estimation of data writing time in Example 1.

MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that the present invention is not limited to the followingdescription, and it will be readily understood by those skilled in theart that modes and details of the present invention can be modified invarious ways without departing from the spirit and scope. Thus, thepresent invention should not be construed as being limited to thedescription in the following embodiments.

Note that in the structures of the invention described below, the samereference numerals are used, in different drawings, for the sameportions or portions having similar functions, and description thereofis omitted. Furthermore, the same hatch pattern is used for the portionshaving similar functions, and the portions are not especially denoted byreference numerals in some cases.

Note that in each drawing described in this specification, the size, thelayer thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, the size, the layer thickness, or theregion is not limited to the illustrated scale.

Note that in this specification and the like, ordinal numbers such as“first” and “second” are used in order to avoid confusion amongcomponents and do not limit the number.

A transistor is a kind of semiconductor element and can achieveamplification of current or voltage, switching operation for controllingconduction or non-conduction, or the like. A transistor in thisspecification includes an IGFET (Insulated Gate Field Effect Transistor)and a thin film transistor (TFT).

Furthermore, functions of a “source” and a “drain” might be switchedwhen a transistor of opposite polarity is employed, a direction ofcurrent flow is changed in circuit operation, or the like. Therefore,the terms “source” and “drain” can be switched in this specification.

In this specification and the like, the terms “source”, “drain”, and“gate” can be replaced with “source electrode”, “drain electrode”, and“gate electrode”, respectively, in some cases.

In this specification and the like, “electrically connected” includesthe case where components are connected through an “object having anyelectric function”. Here, there is no particular limitation on the“object having any electric function” as long as electric signals can betransmitted and received between the connected components. Examples ofan “object having any electric function” are a switching element such asa transistor, a resistor, a coil, a capacitor, and an element with avariety of functions as well as an electrode and a wiring.

In this specification and the like, a display panel that is oneembodiment of the display device has a function of displaying(outputting) an image or the like on (to) a display surface. Hence, thedisplay panel is one embodiment of an output device.

In this specification and the like, a structure in which a connectorsuch as an FPC (Flexible Printed Circuit) or a TCP (Tape CarrierPackage) is attached to a substrate of a display panel, or a structurein which an IC is mounted on a substrate by a COG (Chip On Glass) methodor the like is referred to as a display panel module or a displaymodule, or simply as a display panel or the like in some cases.

In this specification and the like, a touch sensor has a function ofsensing contact, press, approach, or the like of an object such as afinger or a stylus. In addition, the touch sensor may have a function ofsensing the positional information. Therefore, the touch sensor is oneembodiment of an input device. For example, the touch sensor can includeone or more sensor elements.

In this specification and the like, a substrate provided with a touchsensor is referred to as a touch sensor panel or simply as a touchsensor or the like in some cases. Furthermore, in this specification andthe like, a structure in which a connector such as an FPC or a TCP isattached to a substrate of a touch sensor panel, or a structure in whichan IC is mounted on a substrate by a COG method or the like is referredto as a touch sensor panel module, a touch sensor module, or a sensormodule, or simply as a touch sensor or the like in some cases.

Note that in this specification and the like, a touch panel that is oneembodiment of the display device has a function of displaying(outputting) an image or the like on (to) a display surface and afunction as a touch sensor capable of sensing contact, press, approach,or the like of an object such as a finger or a stylus with, on, or tothe display surface. Therefore, the touch panel is one embodiment of aninput/output device.

A touch panel can also be referred to as, for example, a display panel(or a display device) with a touch sensor or a display panel (or adisplay device) having a touch sensor function.

A touch panel can include a display panel and a touch sensor panel.Alternatively, a touch panel can have a function of a touch sensorinside a display panel or on a surface thereof

In this specification and the like, a structure in which a connectorsuch as an FPC or a TCP is attached to a substrate of a touch panel, ora structure in which an IC is mounted on a substrate by a COG method orthe like is referred to as a touch panel module or a display module, orsimply as a touch panel or the like in some cases.

Embodiment 1

In this embodiment, a display device of one embodiment of the presentinvention is described.

One embodiment of the present invention is a display device including adisplay portion where a plurality of pixels are arranged in a matrix. Inthe display portion, a plurality of wirings to which a selection signalis supplied (also referred to as gate lines or scan lines) and aplurality of wirings to which a signal to be written to a pixel (alsoreferred to as a video signal or the like) is supplied (also referred toas source lines, signal lines, data lines, or the like) are provided.Here, the gate lines are provided parallel to one another, the sourcelines are provided parallel to one another, and the gate lines and thesource lines intersect with each other.

One pixel includes at least one transistor and one display element. Thedisplay element includes a conductive layer having a function as a pixelelectrode, and the conductive layer is electrically connected to one ofa source and a drain of the transistor. A gate of the transistor iselectrically connected to a gate line, and the other of the source andthe drain is electrically connected to a source line.

Here, a direction in which the gate lines extend is called a rowdirection or a first direction, and a direction in which the sourcelines extend is called a column direction or a second direction.

Here, three or more adjacent gate lines are preferably supplied with thesame selection signal. That is, selection periods of these gate linesare preferably the same. In particular, four gate lines are preferablyregarded as a group, in which case the structure of a driver circuit canbe simplified.

In the case where the same selection signal is supplied to four gatelines, four pixels which are adjacent to each other in the columndirection are concurrently selected. Thus, a structure in whichdifferent source lines are connected to the four pixels is employed.That is, a structure in which four source lines are arranged for eachcolumn is employed.

With the structure in which four source lines are arranged for eachcolumn, one horizontal period can be longer than the conventional one.For example, in the case where four gate lines are supplied with thesame selection signal, the length of one horizontal period can bequadrupled. Furthermore, since the parasitic capacitance between thesource lines can be reduced, the load of the source lines can bereduced. Thus, even an extremely high-resolution display device with aresolution of 4K, 8K, or the like can be operated with use of atransistor with a low field-effect mobility. Needless to say, thestructure of one embodiment of the present invention even enables theoperation of a display device with a resolution exceeding 8K (forexample, 10K, 12K, or 16K). The above-described structure can also beapplied to a large display device with a diagonal screen size of 50inches or larger, 60 inches or larger, or 70 inches or larger.

In the case where the structure in which four source lines are arrangedfor each column is employed, two source lines can be arranged on theleft side of a pixel and two source lines can be arranged on the rightside of the pixel. In other words, a source line can be provided on eachof the outer left, inner left, inner right, and outer right sides of thepixel. In this structure, a source of a transistor which is electricallyconnected to the source line on the outer left side of the pixelintersects with the source line on the inner left side of the pixel.Also in this structure, a source of a transistor which is electricallyconnected to the source line on the outer right side of the pixelintersects with the source line on the inner right side of the pixel. Inone embodiment of the present invention, a short-circuit between thesource of the transistor which is electrically connected to the sourceline on the outer left side of the pixel and the source line on theinner left side of the pixel is suppressed with use of a conductivelayer that can be formed in the same step as a pixel electrode and aconductive layer that can be formed in the same step as the gate of thetransistor. Also in one embodiment of the present invention, ashort-circuit between the source of the transistor which is electricallyconnected to the source line on the outer right side of the pixel andthe source line on the inner right side of the pixel is suppressed withuse of the conductive layer that can be formed in the same step as thepixel electrode and the conductive layer that can be formed in the samestep as the gate of the transistor. Thus, even in the case of thestructure in which four source lines are arranged for each column, anincrease in the number of steps, specifically, the number ofphotolithography steps, from that in the case of the structure in whichone or two source lines are arranged for each column can be suppressed.In other words, an increase in the number of photomasks can besuppressed. Accordingly, an increase in the manufacturing cost of thedisplay device can be suppressed.

More specific examples of the display device are described below withreference to drawings.

[Structure Example of Display Device]

FIG. 1 is a block diagram of a display device 10 of one embodiment ofthe present invention. The display device 10 includes a display portion17, a gate driver 12 a, a gate driver 12 b, a source driver 13 a, and asource driver 13 b. In the display portion 17, pixels 11 are arranged inmatrix. Note that in this specification and the like, the pixel 11 inthe i-th row and the j-th column is referred to as a pixel 11(i, j).

FIG. 1 shows an example in which the gate driver 12 a and the gatedriver 12 b are provided to face each other with the display portion 17therebetween. A plurality of wirings GL₀ are connected to the gatedriver 12 a and the gate driver 12 b. In FIG. 1, a wiring GL₀(i) isillustrated. The wiring GL₀(i) is electrically connected to four wirings(a wiring GL(i), a wiring GL(i+1), a wiring GL(i+2), and a wiringGL(i+3)). Accordingly, these four wirings are supplied with the sameselection signal. Note that the wiring GL₀ and the wirings GL functionas gate lines.

The gate driver 12 a and the gate driver 12 b have a function ofsupplying the same selection signal to the same wiring GL₀. Accordingly,the charge and discharge time of the wiring GL₀ can be shorter than thatin the case of the display device 10 including only one gate driver.Thus, even an extremely high-resolution display device with a resolutionof 4K, 8K, or the like can be operated with use of a transistor with alow field-effect mobility. In addition, the display device of oneembodiment of the present invention can also be applied to a largedisplay device with a diagonal screen size of 50 inches or larger, 60inches or larger, or 70 inches or larger.

FIG. 1 shows an example in which the source driver 13 a and the sourcedriver 13 b are provided with the display portion 17 therebetween. Aplurality of wirings are connected to the source driver 13 a and thesource driver 13 b. Four wirings are provided for one pixel column. FIG.1 illustrates four wirings (a wiring SL₁(j), a wiring SL₂(j), a wiringSL₃(j), and a wiring SL₄(j)) for the j-th pixel column and four wirings(a wiring SL₁(j+1), a wiring SL₂(j+1), a wiring SL₃(j+1), and a wiringSL₄(j+1)) for the (j+1)-th pixel column. Different signals can besupplied to different wirings. For example, different signals can besupplied to the wiring SL₁(j), the wiring SL₂(j), the wiring SL₃(j), andthe wiring SL₄(j). Note that the wirings SL (the wirings SL₁, thewirings SL₂, the wirings SL₃, and the wirings SL₄) function as sourcelines.

The source driver 13 a and the source driver 13 b have a function ofsupplying the same signal to the same wiring SL. Accordingly, the chargeand discharge time of the wiring SL can be shorter than that in the caseof the display device 10 including only one source driver. Thus, even anextremely high-resolution display device with a resolution of 4K, 8K, orthe like can be operated with use of a transistor with a lowfield-effect mobility. In addition, the display device of one embodimentof the present invention can also be applied to a large display devicewith a diagonal screen size of 50 inches or larger, 60 inches or larger,or 70 inches or larger.

One pixel 11 is a pixel corresponding to one color. Therefore, in thecase where color display is performed by utilizing color mixture oflight emitted from a plurality of pixels, the pixel 11 can be called asub-pixel.

Furthermore, a plurality of pixels arranged in one column in the columndirection are preferably pixels emitting light of the same color. In thecase where liquid crystal elements are used as display elements, thepixels arranged in one column in the column direction are provided withcoloring layers that transmit light of the same color to overlap withthe liquid crystal elements.

Here, in the case where a transistor with a low field-effect mobility isused, a display portion of a display device can be divided into aplurality of display regions and driven. However, in the case of theabove method, a boundary portion between the divided display regionsmight be visually recognized because of, for example, variation incharacteristics of driver circuits, which decreases the visibility insome cases. In addition, image processing or the like for dividing inadvance image data to be input is necessary; thus, a large-scale imageprocessing device that can operate at a high speed is required.

By contrast, the display device of one embodiment of the presentinvention can be driven without dividing the display portion into aplurality of display regions even in the case where a transistor with arelatively low field-effect mobility is used.

A protective circuit may be provided in the display device 10. FIG. 2 isa block diagram showing the case where a protective circuit 18 a, aprotective circuit 18 b, a protective circuit 19 a, and a protectivecircuit 19 b are provided in the display device 10 having the structurein FIG. 1. The protective circuit 18 a and the protective circuit 18 bare electrically connected to the wirings GL₀. The protective circuit 19a and the protective circuit 19 b are electrically connected to thewirings SL₁, the wirings SL₂, the wirings SL₃, and the wirings SL₄.

The protective circuit 18 a can be provided on the gate driver 12 aside, and the protective circuit 18 b can be provided on the gate driver12 b side. In other words, the protective circuit 18 a and theprotective circuit 18 b can be provided to face each other with thedisplay portion 17 therebetween. The protective circuit 19 a can beprovided on the source driver 13 a side, and the protective circuit 19 bcan be provided on the source driver 13 b side. In other words, theprotective circuit 19 a and the protective circuit 19 b can be providedto face each other with the display portion 17 therebetween.

With the protective circuits provided in the display device 10, thepixels 11 can be protected from noise, surge, electrostatic discharge,and the like. This leads to higher reliability of the display device 10.

Although four source lines are provided for one pixel column in FIG. 1,one embodiment of the present invention is not limited to this. FIG. 3illustrates a structure in which three source lines (the wiring SL₁, thewiring SL₂, and the wiring SL₃) are provided for one pixel column. Inthe display device 10 having the structure, the wiring GL₀(i) iselectrically connected to three wirings (the wiring GL(i), the wiringGL(i+1), and the wiring GL(i+2)), and the same selection signal issupplied to these three wirings. Note that five or more source lines maybe provided for one pixel column in one embodiment of the presentinvention.

Although two gate drivers and two source drivers are placed in theexample in FIG. 1, a structure in which the number of gate driversand/or source drivers is one may be employed.

FIG. 4 shows an example in which one source driver 13 a and one sourcedriver 13 b are provided for each pixel column. In other words, thesource drivers 13 a equal in number to the pixel columns are providedalong one side of the rectangular display portion 17, and the sourcedrivers 13 b equal in number to the pixel columns are provided to facethe source drivers 13 a with the display portion 17 therebetween. In theexample in FIG. 4, one gate driver 12 a and one gate driver 12 b areprovided for each wiring GL₀. In other words, the gate drivers 12 awhose number is obtained by dividing the number of pixel rows by 4 areprovided along one side of the rectangular display portion 17, and thegate drivers 12 b whose number is obtained by dividing the number ofpixel rows by 4 are provided to face the gate drivers 12 a with thedisplay portion 17 therebetween. With such a structure, displayunevenness due to a potential drop caused by wiring resistance can besuppressed even in a large display device.

In the display device 10, a reference voltage generation circuit can beprovided. The reference voltage generation circuit has a function ofgenerating a reference voltage of signals supplied by the sourcedrivers. The reference voltage generation circuit can be a gammareference generation circuit, for example. FIG. 5 shows the case where areference voltage generation circuit 16 a having a function of supplyinga reference voltage to the source drivers 13 a and a reference voltagegeneration circuit 16 b having a function of supplying a referencevoltage to the source drivers 13 b are provided in the display device 10having the structure illustrated in FIG. 4. The display device 10 havingthe structure illustrated in FIG. 5 can improve the accuracy of avoltage of a signal generated by each source driver 13 a and theaccuracy of a voltage of a signal generated by each source driver 13 b.

FIG. 6 shows the case where a reference voltage generation circuit 16having a function of supplying a reference voltage to the source drivers13 a and the source drivers 13 b is provided in the display device 10having the structure illustrated in FIG. 4. The display device 10 evenhaving the structure illustrated in FIG. 6 can improve the accuracy of avoltage of a signal generated by each source driver 13 a and theaccuracy of a voltage of a signal generated by each source driver 13 b.

[Structure Example of Pixel]

A structure example of the pixels arranged in the display portion 17 ofthe display device 10 is described below.

FIG. 7 illustrates a circuit diagram including four pixels, the pixel11(i, j), a pixel 11(1+1, j), a pixel 11(1+2, j), and a pixel 11(1+3,j), arranged in one column in the column direction.

One pixel 11 includes a transistor 30, a liquid crystal element 20, anda capacitor 60.

A wiring S1 to a wiring S4 correspond to source lines, and a wiring G1to a wiring G4 correspond to gate lines. For example, in the caseillustrated in FIG. 7, the wiring S1 corresponds to the wiring SL₁(j),the wiring S2 corresponds to the wiring SL₂(j), the wiring S3corresponds to the wiring SL₃(j), and the wiring S4 corresponds to thewiring SL₄(j). In the case illustrated in FIG. 7, the wiring G1corresponds to the wiring GL(i), the wiring G2 corresponds to the wiringGL(i+1), the wiring G3 corresponds to the wiring GL(i+2), and the wiringG4 corresponds to the wiring GL(i+3).

The wiring S1 is electrically connected to one of a source and a drainof the transistor 30 included in the pixel 11(i, j), and the wiring G1is electrically connected to a gate of the transistor 30 included in thepixel 11(i, j). The wiring S2 is electrically connected to one of asource and a drain of the transistor 30 included in the pixel 11(i+1,j), and the wiring G2 is electrically connected to a gate of thetransistor 30 included in the pixel 11(i+1, j). The wiring S3 iselectrically connected to one of a source and a drain of the transistor30 included in the pixel 11(i+2, j), and the wiring G3 is electricallyconnected to a gate of the transistor 30 included in the pixel 11(1+2,j). The wiring S4 is electrically connected to one of a source and adrain of the transistor 30 included in the pixel 11(i+3, j), and thewiring G4 is electrically connected to a gate of the transistor 30included in the pixel 11(i+3, j).

The other of the source and the drain of the transistor 30 iselectrically connected to one electrode of the capacitor 60 and oneelectrode (pixel electrode) of the liquid crystal element 20. The otherelectrode of the capacitor 60 is electrically connected to a wiring CS,and a common potential is supplied to the wiring CS.

The transistor 30 has a function of controlling writing of a signalsupplied from the source line to the pixel 11 by switching an on stateand an off state. Specifically, by turning on the transistor 30, chargecorresponding to the signal supplied from the source line can be writtento the capacitor 60 electrically connected to the transistor 30. Byturning off the transistor 30, the charge written to the capacitor 60can be retained.

Here, the transistor 30 can be a transistor using amorphous silicon. Itis difficult to increase the field-effect mobility of the transistorusing amorphous silicon; however, even when such a transistor is used,the display device of one embodiment of the present invention can haveextremely high resolution such as 4K, 8K, or the like. In addition, alarge display device with a diagonal screen size of 50 inches or larger,60 inches or larger, or 70 inches or larger can be manufactured.

Alternatively, a transistor including a metal oxide in a channelformation region (hereinafter also referred to as an OS transistor) canbe used as the transistor 30. A metal oxide has a larger energy gap thana semiconductor such as silicon, and an OS transistor can have a lowerminority carrier density. Therefore, when an OS transistor is in an offstate, current flowing between a source and a drain of the OS transistor(hereinafter also referred to as off-state current) is extremely low.Thus, when the OS transistor is used as the transistor 30, charge can beretained in the capacitor 60 for a long time. Accordingly, the frequencyof writing of charge to the capacitor 60, that is, the frequency of arefresh operation can be reduced, leading to reduced power consumptionof the display device 10.

In this specification and the like, a metal oxide means an oxide ofmetal in a broad expression. Metal oxides are classified into an oxideinsulator, an oxide conductor (including a transparent oxide conductor),an oxide semiconductor (also simply referred to as an OS), and the like.For example, in the case where a metal oxide is used in a semiconductorlayer of a transistor, the metal oxide is referred to as an oxidesemiconductor in some cases. That is, in the case where a metal oxidehas at least one of an amplifying function, a rectifying function, and aswitching function, the metal oxide can be referred to as a metal oxidesemiconductor, or OS for short. An OS FET refers to a transistorincluding a metal oxide or an oxide semiconductor.

In this specification and the like, a metal oxide containing nitrogen isalso called a metal oxide in some cases. Moreover, a metal oxidecontaining nitrogen may be referred to as a metal oxynitride.

In this specification and the like, CAAC (c-axis aligned crystal) andCAC (Cloud-Aligned Composite) are stated in some cases. Note that CAACrefers to an example of a crystal structure, and CAC refers to anexample of a function or a material composition.

In this specification and the like, a CAC-OS or a CAC-metal oxide has aconducting function in part of the material and an insulating functionin another part of the material, and has a function of a semiconductoras the whole material. Note that in the case where the CAC-OS or theCAC-metal oxide is used in an active layer of a transistor, theconducting function is to allow electrons (or holes) serving as carriersto flow, and the insulating function is to not allow electrons servingas carriers to flow. By the complementary action of the conductingfunction and the insulating function, a switching function (On/Offfunction) can be given to the CAC-OS or the CAC-metal oxide. In theCAC-OS or the CAC-metal oxide, separation of these functions canmaximize both functions.

In this specification and the like, the CAC-OS or the CAC-metal oxideincludes conductive regions and insulating regions. The conductiveregions have the aforementioned conducting function, and the insulatingregions have the aforementioned insulating function. In some cases, theconductive regions and the insulating regions in the material areseparated at the nanoparticle level. In some cases, the conductiveregions and the insulating regions are unevenly distributed in thematerial. The conductive regions are sometimes observed to be coupled ina cloud-like manner with their boundaries blurred.

In the CAC-OS or the CAC-metal oxide, the conductive regions and theinsulating regions each having a size greater than or equal to 0.5 nmand less than or equal to 10 nm, preferably greater than or equal to 0.5nm and less than or equal to 3 nm are dispersed in the material in somecases.

The CAC-OS or the CAC-metal oxide is composed of components havingdifferent bandgaps. For example, the CAC-OS or the CAC-metal oxide iscomposed of a component having a wide gap due to the insulating regionand a component having a narrow gap due to the conductive region. Whencarriers flow in this composition, carriers mainly flow in the componenthaving a narrow gap. Moreover, the component having a narrow gapcomplements the component having a wide gap, and carriers also flow inthe component having a wide gap in conjunction with the component havinga narrow gap. Therefore, in the case where the above-described CAC-OS orCAC-metal oxide is used in a channel formation region of a transistor,the transistor in an on state can achieve high current drivingcapability, that is, a high on-state current and a high field-effectmobility.

In other words, the CAC-OS or the CAC-metal oxide can also be referredto as a matrix composite or a metal matrix composite.

FIG. 8(A) illustrates a layout example of the pixel 11(1+2, j) and thepixel 11(1+3, j).

In FIG. 8(A) and the like, components provided in the same layer areillustrated with the same hatching. Also in the drawings referred tobelow, components provided in the same layer may be illustrated with thesame hatching.

As illustrated in FIG. 8(A), the wiring G3, the wiring G4, and thewiring CS extend in the row direction (the lateral direction), and thewiring S1 to the wiring S4 extend in the column direction (thelongitudinal direction).

A structure example of the pixel 11(i+2, j) is described. In thetransistor 30 included in the pixel 11(i+2, j), a semiconductor layer 32is provided over the wiring G3, and part of the wiring G3 has a functionas the gate. Part of the wiring S3 has a function as one of the sourceand the drain. The semiconductor layer 32 includes a region positionedbetween the wiring S2 and the wiring S3.

A conductive layer 33 a having a function as the other of the source andthe drain of the transistor 30 and as the one electrode of the capacitor60 is provided to be electrically connected to the semiconductor layer32. A conductive layer 21 having a function as the pixel electrode isprovided, and the conductive layer 33 a and the conductive layer 21 areelectrically connected to each other through an opening portion 38.

A structure example of the pixel 11(i+3, j) is described. In thetransistor 30 included in the pixel 11(i+3, j), the semiconductor layer32 is provided over the wiring G4, and part of the wiring G4 has afunction as the gate. The semiconductor layer 32 includes a regionpositioned between the wiring S2 and the wiring S3.

The conductive layer 33 a having a function as the other of the sourceand the drain of the transistor 30 and as the one electrode of thecapacitor 60 is provided to be electrically connected to thesemiconductor layer 32. The conductive layer 21 having a function as thepixel electrode is provided, and the conductive layer 33 a and theconductive layer 21 are electrically connected to each other through theopening portion 38.

A conductive layer 51 having a function as one of the source and thedrain of the transistor 30 is provided to be electrically connected tothe semiconductor layer 32. Through an opening portion 71, theconductive layer 51 is electrically connected to a conductive layer 52formed in the same layer as the conductive layer 21. Through an openingportion 72, the conductive layer 52 is electrically connected to aconductive layer 53 formed in the same layer as the wiring G4. Throughan opening portion 73, the conductive layer 53 is electrically connectedto a conductive layer 54 formed in the same layer as the conductivelayer 21. Through an opening portion 74, the conductive layer 54 iselectrically connected to the wiring S4.

In other words, in the pixel 11(i+3, j), the conductive layer 51 havinga function as one of the source and the drain of the transistor 30 iselectrically connected to the wiring S4 through the conductive layer 52,the conductive layer 53, and the conductive layer 54. In the case wherethe pixel 11(i+3, j) has the structure illustrated in FIG. 8(A), theconductive layer 51, the wiring S3, and the wiring S4 are provided inthe same layer and the conductive layer 53 includes a region overlappingwith the wiring S3; however, a short-circuit between one of the sourceand the drain of the transistor 30 and the wiring S3 can be suppressed.Furthermore, the conductive layer 52 and the conductive layer 54 can beformed in the same step as the conductive layer 21 having a function asthe pixel electrode, and the conductive layer 53 can be formed in thesame step as the wiring G4. Thus, even in the case of the structure inwhich four source lines are arranged for each column, an increase in thenumber of steps, specifically, the number of photolithography steps,from that in the case of the structure in which one or two source linesare arranged for each column can be suppressed. In other words, anincrease in the number of photomasks can be suppressed. Accordingly, anincrease in the manufacturing cost of the display device can besuppressed.

FIG. 8(B) illustrates a layout example of the pixel 11(i, j) and thepixel 11(1+1, j). As illustrated in FIG. 8(B), the wiring G1 and thewiring G2 extend in the row direction.

In the pixel 11(i, j), the conductive layer 51 having a function as oneof the source and the drain of the transistor 30 is electricallyconnected to the wiring S1 through the conductive layer 52, theconductive layer 53, and the conductive layer 54. Except for this, thestructure of the pixel 11(i, j) and the structure of the pixel 11(1+3,j) are the same.

In the pixel 11(1+1, j), part of the wiring S2 has a function as one ofthe source and the drain of the transistor 30. Except for this, thestructure of the pixel 11(1+1, j) and the structure of the pixel 11(1+2,j) are the same.

The above is the description of the structure examples of the pixels.

CROSS-SECTIONAL STRUCTURE EXAMPLE

Examples of a cross-sectional structure of the display device aredescribed below.

Cross-Sectional Structure Example 1

FIG. 9 illustrates an example of a cross section along cutting lineA1-A2 in FIG. 8(A). Here, an example of the case where the transmissiveliquid crystal element 20 is used as a display element is shown. In FIG.9, a substrate 15 side is a display surface side.

The display device 10 has a structure in which a liquid crystal 22 isprovided between a substrate 14 and the substrate 15. The liquid crystalelement 20 includes the conductive layer 21 provided on the substrate 14side, a conductive layer 23 provided on the substrate 15 side, and theliquid crystal 22 provided therebetween. Furthermore, an alignment film24 a is provided between the liquid crystal 22 and the conductive layer21 and an alignment film 24 b is provided between the liquid crystal 22and the conductive layer 23.

The conductive layer 21 has a function as a pixel electrode. Theconductive layer 23 has a function as a common electrode or the like.The conductive layer 21 and the conductive layer 23 each have a functionof transmitting visible light. Thus, the liquid crystal element 20 is atransmissive liquid crystal element.

A coloring layer 41 and a light-blocking layer 42 are provided on asurface of the substrate 15 on the substrate 14 side. An insulatinglayer 26 is provided to cover the coloring layer 41 and thelight-blocking layer 42, and the conductive layer 23 is provided tocover the insulating layer 26. The coloring layer 41 is provided in aregion overlapping with the conductive layer 21. The light-blockinglayer 42 is provided to cover the transistor 30, the opening portion 38,and the like.

A polarizing plate 39 a is located outward from the substrate 14, and apolarizing plate 39 b is located outward from the substrate 15.Furthermore, a backlight unit 90 is located outward from the polarizingplate 39 a.

The transistor 30, the capacitor 60, and the like are provided over thesubstrate 14. The transistor 30 has a function as a selection transistorof the pixel 11. The transistor 30 is electrically connected to theliquid crystal element 20 through the opening portion 38.

The transistor 30 illustrated in FIG. 9 is a transistor that has what iscalled a channel-etched bottom-gate structure. The transistor 30includes a conductive layer 31 having a function as a gate, aninsulating layer 34 having a function as a gate insulating layer, thesemiconductor layer 32, a pair of impurity semiconductor layers 35having a function as a source region and a drain region, and a pair ofthe conductive layer 33 a and a conductive layer 33 b having a functionas a source and a drain. A region of the semiconductor layer 32overlapping with the conductive layer 31 has a function as a channelformation region. The impurity semiconductor layers 35 are provided incontact with the semiconductor layer 32, and the conductive layer 33 aand the conductive layer 33 b are provided in contact with the impuritysemiconductor layers 35.

In this specification and the like, an impurity semiconductor layer issimply referred to as a semiconductor layer in some cases.

Note that the conductive layer 31 corresponds to part of the wiring G3in FIG. 8(A), and the conductive layer 33 b corresponds to part of thewiring S3. Furthermore, a conductive layer 31 a and a conductive layer33 c, which are described later, correspond to part of the wiring CS andpart of the wiring S4, respectively.

A semiconductor containing silicon is preferably used for thesemiconductor layer 32. For example, amorphous silicon, microcrystallinesilicon, polycrystalline silicon, or the like can be used. Inparticular, amorphous silicon can be formed over a large substrate witha high yield, which is preferable. The display device of one embodimentof the present invention can perform favorable display even in the casewhere a transistor including amorphous silicon having a relatively lowfield-effect mobility is used.

The impurity semiconductor layers 35 are formed using a semiconductor towhich an impurity element imparting one conductivity type is added. Inthe case where the transistor is an n-channel transistor, for example,silicon to which P or As is added is given as the semiconductor to whichthe impurity element imparting one conductivity type is added. Incontrast, in the case where the transistor is a p-channel transistor,for example, it is possible to add B as the impurity element impartingone conductivity type; however, it is preferable that the transistor bean n-channel transistor. Note that the impurity semiconductor layers 35may be formed using an amorphous semiconductor or may be formed using acrystalline semiconductor such as a microcrystalline semiconductor.

The capacitor 60 is made of the conductive layer 31 a, the insulatinglayer 34, and the conductive layer 33 a. Furthermore, the conductivelayer 33 c is provided over the conductive layer 31 with the insulatinglayer 34 therebetween.

An insulating layer 82 and an insulating layer 81 are stacked to coverthe transistor 30 and the like. The conductive layer 21 having afunction as the pixel electrode is provided over the insulating layer81. The conductive layer 21 and the conductive layer 33 a areelectrically connected to each other through the opening portion 38provided in the insulating layer 81 and the insulating layer 82. Theinsulating layer 81 preferably has a function as a planarization layer.The insulating layer 82 preferably has a function as a protective filmthat inhibits diffusion of impurities or the like to the transistor 30and the like. An inorganic insulating material can be used for theinsulating layer 82, and an organic insulating material can be used forthe insulating layer 81, for example.

In this specification and the like, the insulating layer 82 and theinsulating layer 81 are collectively regarded as one insulating layer insome cases.

Cross-Sectional Structure Example 2

FIG. 10 illustrates an example of a cross section along cutting lineB1-B2 in FIG. 8(A). The transistor 30 illustrated in FIG. 10 includesthe conductive layer 31 having a function as a gate, the insulatinglayer 34 having a function as a gate insulating layer, the semiconductorlayer 32, the pair of impurity semiconductor layers 35 having a functionas a source region and a drain region, and a pair of the conductivelayer 33 a and the conductive layer 51 having a function as a source anda drain. A region of the semiconductor layer 32 overlapping with theconductive layer 31 has a function as a channel formation region. Theimpurity semiconductor layers 35 are provided in contact with thesemiconductor layer 32, and the conductive layer 33 a and the conductivelayer 51 are provided in contact with the impurity semiconductor layers35.

Note that the conductive layer 31 corresponds to part of the wiring G4in FIG. 8(A). As in the case illustrated in FIG. 9, the conductive layer31 a, the conductive layer 33 b, and the conductive layer 33 ccorrespond to part of the wiring CS, part of the wiring S3, and part ofthe wiring S4, respectively. The conductive layer 33 b is provided tohave a region overlapping with the conductive layer 53 with theinsulating layer 34 therebetween.

As described above, the conductive layer 51 and the conductive layer 52are electrically connected to each other through the opening portion 71provided in the insulating layer 81 and the insulating layer 82. Theconductive layer 52 and the conductive layer 53 are electricallyconnected to each other through the opening portion 72 provided in theinsulating layer 81, the insulating layer 82, and the insulating layer34. The conductive layer 53 and the conductive layer 54 are electricallyconnected to each other through the opening portion 73 provided in theinsulating layer 81, the insulating layer 82, and the insulating layer34. The conductive layer 54 and the conductive layer 33 c areelectrically connected to each other through the opening portion 74provided in the insulating layer 81 and the insulating layer 82. Thatis, as described above, the conductive layer 51 having a function as oneof the source and the drain of the transistor 30 is electricallyconnected to the conductive layer 33 c corresponding to part of thewiring S4 through the conductive layer 52, the conductive layer 53, andthe conductive layer 54. The opening portion 72 and the opening portion73 are formed with the conductive layer 33 b therebetween. Accordingly,a short-circuit between the conductive layer 51 having a function as oneof the source and the drain of the transistor 30 and the conductivelayer 33 b corresponding to part of the wiring S3 is suppressed. Notethat as illustrated in FIG. 10, the conductive layer 52 and theconductive layer 54 are formed in the same layer as the conductive layer21, and the conductive layer 53 is formed in the same layer as theconductive layer 31 and the conductive layer 31 a.

Note that components formed in the same layer can include the samematerial. In other words, for example, the conductive layer 21, theconductive layer 52, and the conductive layer 54 can include the samematerial. For example, the conductive layer 31, the conductive layer 31a, and the conductive layer 53 can include the same material.

Cross-Sectional Structure Example 3

FIG. 11 illustrates a modification example of the structure illustratedin FIG. 10. FIG. 11 illustrates an example of the case where thecoloring layer 41 is provided on the substrate 14 side. Thus, thestructure on the substrate 15 side can be simplified.

Note that in the case where the coloring layer 41 is used as aplanarization film, a structure in which the insulating layer 81 is notprovided may be employed. Thus, the number of manufacturing steps of thedisplay device 10 can be reduced, and the manufacturing cost of thedisplay device 10 can be reduced.

Cross-Sectional Structure Example 4

FIG. 12 illustrates a modification example of the structure illustratedin FIG. 10. In FIG. 12, an example of the case where the conductivelayer 52, the conductive layer 53, the conductive layer 54, the openingportion 72, and the opening portion 73 are omitted. In this case, theconductive layer 51 and the conductive layer 33 c are electricallyconnected to each other through a conductive layer 55 formed in the samelayer as the conductive layer 21. Specifically, the conductive layer 51and the conductive layer 55 are electrically connected to each otherthrough the opening portion 71, and the conductive layer 33 c and theconductive layer 55 are electrically connected to each other through theopening portion 74. In the case where the structure illustrated in FIG.12 is employed, a short-circuit between the conductive layer 51 and theconductive layer 33 b can be suppressed.

Cross-Sectional Structure Example 5>

FIG. 13, FIG. 14, FIG. 15, and FIG. 16 illustrate a modification exampleof the structure illustrated in FIG. 9, a modification example of thestructure illustrated in FIG. 10, a modification example of thestructure illustrated in FIG. 11, and a modification example of thestructure illustrated in FIG. 12, respectively. The structuresillustrated in FIG. 13 to FIG. 16 are different from the structuresillustrated in FIG. 9 to FIG. 12 in that the impurity semiconductorlayers 35 are not included.

In the structures illustrated in FIG. 13 to FIG. 16, it is preferable touse a semiconductor including a metal oxide for the semiconductor layer32. A semiconductor including a metal oxide is used for thesemiconductor layer 32, that is, an OS transistor is used as thetransistor 30, whereby charge corresponding to the signal supplied fromthe source line can be retained in the capacitor 60 for a long time, asdescribed above. Accordingly, the frequency of writing of charge to thecapacitor 60, that is, the frequency of a refresh operation can bereduced, leading to reduced power consumption of the display device 10.

The above is the description of the cross-sectional structure examples.

[Components]

Each of the above-described components is described below.

<Substrate>

A material having a flat surface can be used as the substrate includedin the display panel. As the substrate through which light from thedisplay element is extracted, a material transmitting the light is used.For example, a material such as glass, quartz, ceramics, sapphire, or anorganic resin can be used.

The weight and thickness of the display panel can be reduced by using asubstrate with a small thickness. Furthermore, a display panel havingflexibility can be obtained by using a substrate that is thin enough tohave flexibility. Alternatively, glass or the like that is thin enoughto have flexibility can be used as the substrate. Further alternatively,a composite material where glass and a resin material are attached toeach other with an adhesive layer may be used.

<Transistor>

The transistor includes a conductive layer having a function as a gate,a semiconductor layer, a conductive layer having a function as a source,a conductive layer having a function as a drain, and an insulating layerhaving a function as a gate insulating layer.

Note that there is no particular limitation on the structure of thetransistor included in the display device of one embodiment of thepresent invention. For example, a planar transistor may be used, astaggered transistor may be used, or an inverted staggered transistormay be used. Either a top-gate transistor or a bottom-gate transistormay be used. Alternatively, gates may be provided above and below achannel.

<Semiconductor Layer>

There is no particular limitation on the crystallinity of thesemiconductor layer used for the transistor, and either an amorphoussemiconductor or a semiconductor having crystallinity (amicrocrystalline semiconductor, a polycrystalline semiconductor, asingle crystal semiconductor, or a semiconductor partly includingcrystal regions) may be used. The use of a semiconductor havingcrystallinity can suppress deterioration of the transistorcharacteristics, which is preferable.

For the semiconductor layer of the transistor, for example, an elementof Group 14 (silicon, germanium, or the like) can be used. In the casewhere silicon is used for the semiconductor layer of the transistor, itis particularly preferable to use amorphous silicon. By using amorphoussilicon, the transistor can be formed over a large substrate with a highyield, so that mass productivity of the display device of one embodimentof the present invention can be increased.

Alternatively, silicon having crystallinity such as microcrystallinesilicon, polycrystalline silicon, or single crystal silicon can be used.In particular, polycrystalline silicon can be formed at a lowertemperature than single crystal silicon and has a higher field-effectmobility and a higher reliability than amorphous silicon.

The bottom-gate transistor described as an example in this embodiment ispreferable because the number of manufacturing steps can be reduced. Inaddition, when amorphous silicon, which can be formed at a lowertemperature than polycrystalline silicon, is used, materials with lowheat resistance can be used as a material of a wiring or an electrode,or a material of a substrate below the semiconductor layer, resulting inwider choice of materials. For example, an extremely large glasssubstrate or the like can be favorably used. Meanwhile, the top-gatetransistor is preferable because an impurity region is easily formed ina self-aligned manner and therefore variation in characteristics and thelike can be reduced. This case is particularly preferable in some caseswhen polycrystalline silicon, single crystal silicon, or the like isused.

A metal oxide can be used for the semiconductor layer of the transistor.Typically, a semiconductor containing silicon, a semiconductorcontaining gallium arsenide, a metal oxide containing indium, or thelike can be used.

In particular, a metal oxide having a wider band gap than silicon ispreferably used. A semiconductor material having a wider band gap and alower carrier density than silicon is preferably used because off-statecurrent of the transistor can be reduced.

Owing to its low off-state current, a transistor using a metal oxidehaving a wider band gap than silicon enables long-term retention ofcharges stored in a capacitor that is series-connected to thetransistor. The use of such a transistor in a pixel allows a drivercircuit to stop while the gray level of an image displayed on eachdisplay portion is maintained. As a result, a display device withextremely low power consumption can be obtained.

The semiconductor layer preferably includes, for example, a filmrepresented by an In-M-Zn-based oxide that contains at least indium,zinc, and M (a metal such as aluminum, titanium, gallium, germanium,yttrium, zirconium, lanthanum, cerium, tin, neodymium, or hafnium). Inorder to reduce variation in electrical characteristics of thetransistors using the semiconductor layer, a stabilizer is preferablycontained in addition to them.

Examples of the stabilizer, including the metals that are describedabove as M, are gallium, tin, hafnium, aluminum, and zirconium. Asanother stabilizer, lanthanoid such as lanthanum, cerium, praseodymium,neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium,erbium, thulium, ytterbium, or lutetium can be given.

As a metal oxide contained in the semiconductor layer, for example, anIn—Ga—Zn-based oxide, an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide,an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-basedoxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, anIn—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide,an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-basedoxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, anIn—Yb—Zn-based oxide, an In—Lu—Zn-based oxide, an In—Sn—Ga—Zn-basedoxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or anIn—Hf—Al—Zn-based oxide can be used.

Note that here, for example, an In—Ga—Zn-based oxide means an oxidecontaining In, Ga, and Zn as its main components and there is nolimitation on the atomic ratio of In to Ga and Zn. For example, theatomic ratio may be In:Ga:Zn=1:1:1, may be In:Ga:Zn=2:2:1, may beIn:Ga:Zn=3:1:2, may be In:Ga:Zn=4:2:3, may be In:Ga:Zn=5:1:6, or may bein the neighborhood thereof. Furthermore, a metal element other than In,Ga, and Zn may be contained.

The semiconductor layer and the conductive layer may include the samemetal elements contained in the above oxides. The use of the same metalelements for the semiconductor layer and the conductive layer can reducethe manufacturing cost. For example, the use of metal oxide targets withthe same metal composition can reduce the manufacturing cost. Inaddition, the same etching gas or the same etchant can be used inprocessing the semiconductor layer and the conductive layer. Note thateven when the semiconductor layer and the conductive layer include thesame metal elements, they have different compositions in some cases. Forexample, a metal element in a film is released during the manufacturingprocess of the transistor and the capacitor, which might result indifferent metal compositions.

The energy gap of the metal oxide contained in the semiconductor layeris greater than or equal to 2 eV, preferably greater than or equal to2.5 eV, further preferably greater than or equal to 3 eV. With use of ametal oxide having such a wide energy gap, the off-state current of thetransistor can be reduced.

In the case where the metal oxide contained in the semiconductor layeris an In-M-Zn oxide, it is preferable that the atomic ratio of metalelements of a sputtering target used for forming a film of the In-M-Znoxide satisfy In≥M. As the atomic ratio of metal elements of such asputtering target, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=3:1:2,4:2:4.1, In:M:Zn=2:1:3, In:M:Zn=3:1:2, In:M:Zn=5:1:6, In:M:Zn=5:1:7,In:M:Zn=5:1:8, In:M:Zn=6:1:6, In:M:Zn=5:2:5, and the like arepreferable. Note that the atomic ratio of metal elements contained inthe formed semiconductor layer varies from the above atomic ratio ofmetal elements contained in the sputtering target within a range of ±40%as an error.

The metal oxide contained in the semiconductor layer is preferably alater-described CAC-OS or CAC-metal oxide. Thus, the field-effectmobility of the transistor can be increased.

A metal oxide with a low carrier density is preferably used for thesemiconductor layer. For example, a metal oxide whose carrier density islower than or equal to 1×10¹⁷/cm³, preferably lower than or equal to1×10¹⁵/cm³, further preferably lower than or equal to 1×10¹³/cm³, stillfurther preferably lower than or equal to 1×10¹¹/cm³, yet furtherpreferably lower than 1×10¹⁰/cm³, and higher than or equal to 1×10⁹/cm³can be used for the semiconductor layer. Such a semiconductor layer hasa low impurity concentration and a low density of defect states and thushas stable characteristics. Note that when the semiconductor layer isthe metal oxide, water, hydrogen, or the like can be given as animpurity.

In this specification and the like, a metal oxide with a low impurityconcentration and a low density of defect states is referred to as ahighly purified intrinsic metal oxide or a substantially highly purifiedintrinsic metal oxide in some cases.

A highly purified intrinsic or substantially highly purified intrinsicmetal oxide has few carrier generation sources, and thus has a lowcarrier density. Thus, a transistor including the metal oxide rarely haselectrical characteristics in which the threshold voltage is negative(also referred to as normally-on). The highly purified intrinsic orsubstantially highly purified intrinsic metal oxide has a low density ofdefect states and accordingly has a low density of trap states in somecases. Furthermore, a transistor including the highly purified intrinsicor substantially highly purified intrinsic metal oxide has an extremelylow off-state current; even when an element has a channel width W of1×10⁶ μm and a channel length L of 10 μm, the off-state current can belower than or equal to the measurement limit of a semiconductorparameter analyzer, that is, lower than or equal to 1×10⁻¹³ A, at avoltage (drain voltage) between a source and a drain in the range from 1V to 10 V.

Note that the semiconductor layer that can be used in one embodiment ofthe present invention is not limited to the above, and a material withan appropriate composition may be used depending on requiredsemiconductor characteristics and electric characteristics (field-effectmobility, threshold voltage, and the like) of a transistor. To obtainthe required semiconductor characteristics of the transistor, it ispreferable that the carrier density, the impurity concentration, thedefect density, the atomic ratio between a metal element and oxygen, theinteratomic distance, the density, and the like of the semiconductorlayer be set to appropriate values.

When silicon or carbon that is one of Group 14 elements is contained inthe metal oxide contained in the semiconductor layer, oxygen vacanciesare increased in the semiconductor layer, and the semiconductor layerbecomes n-type in some cases. Thus, the concentration of silicon orcarbon (the concentration measured by secondary ion mass spectrometry)in the semiconductor layer is preferably lower than or equal to 2×10¹⁸atoms/cm³, further preferably lower than or equal to 2×10¹⁷ atoms/cm³.

An alkali metal and an alkaline earth metal generate carriers in somecases when bonded to a metal oxide, in which case the off-state currentof the transistor might be increased. Therefore, the concentration of analkali metal or an alkaline earth metal of the semiconductor layer,which is obtained by secondary ion mass spectrometry, is preferablylower than or equal to 1×10¹⁸ atoms/cm³, further preferably lower thanor equal to 2×10¹⁶ atoms/cm³.

The semiconductor layer may have a non-single-crystal structure, forexample. Non-single-crystal structures include a polycrystallinestructure, a microcrystalline structure, and an amorphous structure, forexample. Among the non-single-crystal structures, the amorphousstructure has the highest density of defect states.

A metal oxide having an amorphous structure has disordered atomicarrangement and no crystalline component, for example. Alternatively, anoxide film having an amorphous structure has, for example, an absolutelyamorphous structure and no crystal part.

Note that the semiconductor layer may be a mixed film including two ormore of a region having an amorphous structure, a region having amicrocrystalline structure, a region having a polycrystalline structure,and a region having a single crystal structure. The mixed film has, forexample, a single-layer structure or a stacked-layer structure includingtwo or more of the above regions in some cases.

<Conductive Layer>

As materials that can be used for the gate, source, and drain of thetransistor, and the conductive layers such as the wirings and electrodesincluded in the display device, metals such as aluminum, titanium,chromium, nickel, copper, yttrium, zirconium, molybdenum, silver,tantalum, and tungsten, an alloy containing any of them as its maincomponent, or the like can be given. A single-layer or stacked-layerstructure including a film containing any of these materials can beused. For example, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which an aluminum film isstacked over a titanium film, a two-layer structure in which an aluminumfilm is stacked over a tungsten film, a two-layer structure in which acopper film is stacked over a copper-magnesium-aluminum alloy film, atwo-layer structure in which a copper film is stacked over a titaniumfilm, a two-layer structure in which a copper film is stacked over atungsten film, a three-layer structure in which an aluminum film or acopper film is stacked over a titanium film or a titanium nitride filmand a titanium film or a titanium nitride film is formed thereover, athree-layer structure in which an aluminum film or a copper film isstacked over a molybdenum film or a molybdenum nitride film and amolybdenum film or a molybdenum nitride film is formed thereover, andthe like can be given. Note that an oxide such as indium oxide, tinoxide, or zinc oxide may be used. Copper containing manganese ispreferably used because it increases controllability of a shape byetching.

As a light-transmitting conductive material that can be used for thegate, source, and drain of the transistor and the conductive layers suchas the wirings and electrodes included in the display device, aconductive oxide such as indium oxide, indium tin oxide, indium zincoxide, zinc oxide, or zinc oxide to which gallium is added, or graphenecan be used. Alternatively, a metal material such as gold, silver,platinum, magnesium, nickel, tungsten, chromium, molybdenum, iron,cobalt, copper, palladium, or titanium, or an alloy material containingthe metal material can be used. Further alternatively, a nitride of themetal material (e.g., titanium nitride) or the like may be used. Notethat in the case of using the metal material or the alloy material (orthe nitride thereof), the thickness is set small enough to be able totransmit light. A stacked film of any of the above materials can be usedfor the conductive layers. For example, when a stacked film of indiumtin oxide and an alloy of silver and magnesium, or the like is used, theconductivity can be increased, which is preferable. They can also beused for the conductive layers such as the wirings and electrodesincluded in the display device, and conductive layers (a conductivelayer having a function as a pixel electrode or a common electrode)included in the display element.

<Insulating Layer>

Examples of an insulating material that can be used for each of theinsulating layers include, in addition to a resin such as acrylic orepoxy and a resin having a siloxane bond, an inorganic insulatingmaterial such as silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, or aluminum oxide.

In the case where the semiconductor layer includes a metal oxide, aninsulating layer including a region in contact with the semiconductorlayer preferably includes a region containing oxygen in excess of thestoichiometric composition (an excess oxygen region). For example, theinsulating layer 34 and the insulating layer 82 including a region incontact with the semiconductor layer 32 preferably include an excessoxygen region. Thus, oxygen can be supplied from the insulating layer 34and the insulating layer 82 to the semiconductor layer 32. In the casewhere the semiconductor layer 32 includes a metal oxide and oxygenvacancies are formed in the metal oxide, impurities such as hydrogenenters the oxygen vacancies and generates an electron serving as acarrier in some cases. This degrades the electrical characteristics ofthe transistor in some cases. In the case where the insulating layerincluding a region in contact with the semiconductor layer includes theexcess oxygen region, oxygen can be supplied from the insulating layerto the semiconductor layer, so that the oxygen vacancies can be filled.Thus, the degradation of the electrical characteristics of thetransistor can be suppressed. Note that in order to provide the excessoxygen region in the insulating layer, the insulating layer may beformed in an oxygen atmosphere, for example. Alternatively, the formedinsulating layer may be subjected to heat treatment in an oxygenatmosphere.

<Liquid Crystal Element>

As the liquid crystal element, for example, a liquid crystal elementemploying a vertical alignment (VA) mode can be used. Examples of thevertical alignment mode include an MVA (Multi-Domain Vertical Alignment)mode, a PVA (Patterned Vertical Alignment) mode, and an ASV (AdvancedSuper View) mode.

As the liquid crystal element, a liquid crystal element employing any ofa variety of modes can be used. For example, it is possible to use aliquid crystal element employing a TN (Twisted Nematic) mode, an IPS(In-Plane-Switching) mode, an FFS (Fringe Field Switching) mode, an ASM(Axially Symmetric aligned Micro-cell) mode, an OCB (OpticallyCompensated Birefringence) mode, an FLC (Ferroelectric Liquid Crystal)mode, an AFLC (AntiFerroelectric Liquid Crystal) mode, an ECB(Electrically Controlled Birefringence) mode, a guest-host mode, or thelike instead of a VA mode.

Note that the liquid crystal element is an element that controls thetransmission or non-transmission of light utilizing an opticalmodulation action of a liquid crystal. Note that the optical modulationaction of the liquid crystal is controlled by an electric field appliedto the liquid crystal (including a horizontal electric field, a verticalelectric field, or an oblique electric field). As the liquid crystalused for the liquid crystal element, a thermotropic liquid crystal, alow-molecular liquid crystal, a high-molecular liquid crystal, a polymerdispersed liquid crystal (PDLC), a polymer network liquid crystal(PNLC), a ferroelectric liquid crystal, an anti-ferroelectric liquidcrystal, or the like can be used. These liquid crystal materials exhibita cholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

As the liquid crystal material, either a positive liquid crystal or anegative liquid crystal may be used, and an optimal liquid crystalmaterial can be used depending on the mode or design to be used.

An alignment film can be provided to control the alignment of a liquidcrystal. In the case where a horizontal electric field mode is employed,a liquid crystal exhibiting a blue phase for which an alignment film isnot used may be used. The blue phase is one of the liquid crystalphases, which appears just before a cholesteric phase changes into anisotropic phase when the temperature of a cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which a chiral material is mixedto account for several weight percent or more is used for the liquidcrystal layer in order to improve the temperature range. The liquidcrystal composition containing a liquid crystal exhibiting a blue phaseand a chiral material has a short response time and optical isotropy. Inaddition, the liquid crystal composition containing a liquid crystalexhibiting a blue phase and a chiral material does not need alignmenttreatment and has small viewing angle dependence. Since the alignmentfilm does not need to be provided, rubbing treatment is not necessary;accordingly, electrostatic discharge damage caused by the rubbingtreatment can be prevented, reducing defects and damage of a liquidcrystal display device in the manufacturing process.

The liquid crystal element may be a transmissive liquid crystal element,a reflective liquid crystal element, a transflective liquid crystalelement, or the like.

In one embodiment of the present invention, a transmissive liquidcrystal element is particularly suitably used.

In the case where a transmissive or transflective liquid crystal elementis used, two polarizing plates are provided such that a pair ofsubstrates are sandwiched therebetween. Furthermore, a backlight isprovided on the outer side of the polarizing plate. The backlight may bea direct-below backlight or may be an edge-light backlight. Thedirect-below backlight including an LED (Light Emitting Diode) ispreferably used because local dimming is easily performed to improvecontrast. The edge-light backlight is preferably used because thethickness of a module including the backlight can be reduced.

When the edge-light backlight is turned off, see-through display can beperformed.

<Coloring Layer>

As a material that can be used for the coloring layer, a metal material,a resin material, a resin material containing a pigment or dye, and thelike can be given.

<Light-Blocking Layer>

As a material that can be used for the light-blocking layer, carbonblack, titanium black, a metal, a metal oxide, a composite oxidecontaining a solid solution of a plurality of metal oxides, and the likecan be given. The light-blocking layer may be a film containing a resinmaterial or may be a thin film of an inorganic material such as a metal.Stacked films of films containing the material of the coloring layer canalso be used for the light-blocking layer. For example, a stacked-layerstructure of a film containing a material used for a coloring layerwhich transmits light of a certain color and a film containing amaterial used for a coloring layer which transmits light of anothercolor can be employed. It is preferable that the coloring layer and thelight-blocking layer be formed using the same material because the sameapparatus can be used and the process can be simplified.

The above is the description of each of the components.

[Example of Manufacturing Method of Pixel and the Like]

An example of a manufacturing method of the display device 10 will bedescribed below.

Thin films included in the display device (an insulating film, asemiconductor film, a conductive film, and the like) can each be formedby a sputtering method, a chemical vapor deposition (CVD) method, avacuum evaporation method, a pulsed laser deposition (PLD) method, anatomic layer deposition (ALD) method, or the like. As examples of theCVD method, a plasma-enhanced chemical vapor deposition (PECVD) method,a thermal CVD method, and the like can be given. As an example of thethermal CVD method, a metal organic chemical vapor deposition (MOCVD)method can be given.

The thin films included in the display device (the insulating film, thesemiconductor film, the conductive film, and the like) can each beformed by a method such as spin coating, dipping, spray coating, inkjetprinting, dispensing, screen printing, or offset printing, or with adoctor knife, a slit coater, a roll coater, a curtain coater, or a knifecoater.

The thin films included in the display device can be processed by alithography method or the like. Alternatively, island-shaped thin filmsmay be formed by a film formation method using a blocking mask. Furtheralternatively, the thin films may be processed by a nano-imprintingmethod, a sandblasting method, a lift-off method, or the like.

In the case of processing by a photolithography method, light with ani-line (a wavelength of 365 nm), light with a g-line (a wavelength of436 nm), light with an h-line (a wavelength of 405 nm), and light inwhich these are mixed can be given as light used in exposure, forexample. Besides, ultraviolet light, KrF laser light, ArF laser light,or the like can also be used. Exposure may be performed by liquidimmersion exposure technique. As the light used in exposure, extremeultra-violet light (EUV), X-rays, and the like can be given. An electronbeam can be used instead of the light used in exposure. It is preferableto use extreme ultra-violet light, X-rays, or an electron beam becauseextremely minute processing can be performed. Note that when exposure isperformed by scanning of a beam such as an electron beam, a photomask isnot needed.

For etching of the thin films, a dry etching method, a wet etchingmethod, a sandblast method, or the like can be used.

Example 1 of Manufacturing Method

FIG. 17 to FIG. 19 illustrate an example of a manufacturing method ofthe pixel 11(i+3, j) and the like having the structure illustrated inFIG. 10. In manufacturing the display device 10, first, a conductivelayer is formed over the substrate 14. Next, patterning is performed bya photolithography method or the like and the conductive layer isprocessed by an etching method or the like, whereby the conductive layer31, the conductive layer 31 a, and the conductive layer 53 are formed(FIG. 17(A)). As described above, the conductive layer 31 corresponds topart of the wiring G3, and the conductive layer 31 a corresponds to partof the wiring CS.

Next, the insulating layer 34 is formed. As described above, theinsulating layer 34 has a function as the gate insulating layer of thetransistor provided in the display device 10.

After that, a semiconductor layer is formed over the insulating layer34. In the case where, for example, amorphous silicon is used for thesemiconductor layer, film formation can be performed by a CVD method orthe like using monosilane or the like as a source material. Thus,dangling bonds of silicon included in the semiconductor layer can beterminated with hydrogen and thermodynamic stability can be obtained.Amorphous silicon containing hydrogen, as described above, is referredto as hydrogenated amorphous silicon.

Next, an impurity semiconductor layer which is a semiconductor layercontaining an impurity is formed over the semiconductor layer. In thecase where, for example, hydrogenated amorphous silicon is used for theimpurity semiconductor layer, when the transistor is an n-typetransistor, film formation can be performed by a CVD method or the likein which phosphine, arsine, or the like is added to a source materialsuch as monosilane. When the transistor is a p-type transistor, theimpurity semiconductor layer can be formed by a CVD method or the likeby adding diborane or the like to a source material such as monosilane.

Then, patterning is performed by a photolithography method or the likeand the formed semiconductor layer is processed by an etching method orthe like, whereby the semiconductor layer 32 and the impuritysemiconductor layer 35 are formed (FIG. 17(B)).

Next, a conductive layer is formed over the insulating layer 34 and overthe impurity semiconductor layer 35. After that, patterning is performedby a photolithography method or the like and the conductive layer isprocessed by an etching method or the like, whereby the conductive layer51, the conductive layer 33 a, the conductive layer 33 b, and theconductive layer 33 c are formed (FIG. 17(C)). As described above, theconductive layer 51 functions as one of the source and the drain of thetransistor 30, and the conductive layer 33 a has a function as the otherof the source and the drain of the transistor 30 and as the oneelectrode of the capacitor 60. Furthermore, the conductive layer 33 bcorresponds to part of the wiring S3 and the conductive layer 33 ccorresponds to part of the wiring S4. The conductive layer 33 b isformed so as to have a region overlapping with the conductive layer 53.

Next, the insulating layer 82 is formed, and then the insulating layer81 is formed. After the insulating layer 81 is formed, planarizationtreatment is performed on the insulating layer 81 by a chemicalmechanical polishing (CMP) method or the like.

Next, patterning is performed by a photolithography method or the like.Then, the insulating layer 81 and the insulating layer 82 are processedby an etching method or the like, whereby the opening portion 71reaching the conductive layer 51, the opening portion 38 reaching theconductive layer 33 a, and the opening portion 74 reaching theconductive layer 33 c are formed. Furthermore, the insulating layer 81,the insulating layer 82, and the insulating layer 34 are processed by anetching method or the like, whereby the opening portion 72 and theopening portion 73 reaching the conductive layer 53 are formed with theconductive layer 33 b therebetween (FIG. 18(A)). Thus, the openingportion 38 and the opening portion 71 to the opening portion 74 areformed.

Next, a conductive layer is formed over the insulating layer 81 and inthe opening portion 38 and the opening portion 71 to the opening portion74. After that, patterning is performed by a photolithography method orthe like and the conductive layer is processed by an etching method orthe like, whereby the conductive layer 21, the conductive layer 52, andthe conductive layer 54 are formed (FIG. 18(B)). The conductive layer 21is electrically connected to the conductive layer 33 a through theopening portion 38. The conductive layer 52 is electrically connected tothe conductive layer 51 through the opening portion 71 and electricallyconnected to the conductive layer 53 through the opening portion 72. Theconductive layer 54 is electrically connected to the conductive layer 53through the opening portion 73 and electrically connected to theconductive layer 33 c through the opening portion 74. As describedabove, the conductive layer 21 has a function as the pixel electrode ofthe liquid crystal element provided in the display device 10. Inaddition, the conductive layer 51 having a function as one of the sourceand the drain of the transistor 30 is electrically connected to theconductive layer 33 c corresponding to part of the wiring S4 through theconductive layer 52, the conductive layer 53, and the conductive layer54.

Next, the alignment film 24 a is formed (FIG. 19(A)). After that, thelight-blocking layer 42, the coloring layer 41, the insulating layer 26,the conductive layer 23, and the alignment film 24 b are formed over thesubstrate 15 (FIG. 19(B)). The coloring layer 41 can be formed by aphotolithography method, a printing method, or an inkjet method. Byusing an inkjet method, for example, the coloring layer 41 can be formedat room temperature, formed at a low vacuum, or formed over a largesubstrate. Thus, the coloring layer 41 can be formed even in anextremely high-resolution display device with a resolution of 4K, 8K, orthe like. The coloring layer 41 can also be formed in a large displaydevice with a diagonal screen size of 50 inches or larger, 60 inches orlarger, or 70 inches or larger. Since the coloring layer 41 can beformed without a resist mask, the number of manufacturing steps of thedisplay device 10 can be reduced and the manufacturing cost can bereduced.

Next, between the substrate 14 illustrated in FIG. 19(A) and thesubstrate 15 illustrated in FIG. 19(B), the liquid crystal 22 is sealedwith an adhesive layer (not illustrated). Then, the polarizing plate 39a, the polarizing plate 39 b, and the backlight unit 90 are formed.Through the above steps, the display device 10 having the structureillustrated in FIG. 10 can be manufactured.

Here, in manufacturing a display device, the smaller the number ofphotolithography steps in a manufacturing process is, i.e., the smallerthe number of photomasks is, the lower the manufacturing cost can be.

For example, among the steps illustrated in FIG. 17 and FIG. 18 (thesteps involved in the substrate 14 side), the display device 10 can bemanufactured through five photolithography steps in total: a formationstep of the conductive layer 31 and the like (FIG. 17(A)), a formationstep of the semiconductor layer 32 and the like (FIG. 17(B)), aformation step of the conductive layer 33 a and the like (FIG. 17(C)), aformation step of the opening portion 38 and the like (FIG. 18(A)), anda formation step of the conductive layer 21 and the like (FIG. 18(B)).That is, a backplane substrate can be manufactured with five photomasks.

In the case where the display device has a structure in which one or twosource lines are provided for each pixel column, the pixel 11 having thestructure illustrated in FIG. 10 is not necessarily provided and all thepixels 11 can have the structure illustrated in FIG. 9, for example.Even in this case, the manufacture of the backplane substrate needs fivephotolithography steps in total. That is, five photomasks are necessary.Thus, even in the case of a structure in which four source lines areprovided for each pixel column, the display device can be manufacturedwith the same number of photomasks as that in the case where one or twosource lines are provided for each pixel column. Accordingly, themanufacturing cost of the display device having the structure in whichfour source lines are provided for each pixel column can be preventedfrom exceeding the manufacturing cost of the display device having thestructure in which one or two source lines are provided for each pixelcolumn.

Example 2 of Manufacturing Method

FIG. 20 to FIG. 22 illustrate an example of a manufacturing method ofthe pixel 11(i+3, j) and the like having the structure illustrated inFIG. 14. FIGS. 20(A), (B), and (C), FIGS. 21(A) and (B), and FIGS. 22(A)and (B) correspond to FIGS. 17(A), (B), and (C), FIGS. 18(A) and (B),and FIGS. 19(A) and (B), respectively. The manufacturing methodillustrated in FIG. 20 to FIG. 22 is different from the above-mentionedmanufacturing method in that the impurity semiconductor layers 35 arenot formed in the step illustrated in FIG. 20(B).

In the manufacturing method illustrated in FIG. 20 to FIG. 22, a metaloxide can be used for the semiconductor layer formed over the insulatinglayer 34, for example. In this case, the semiconductor layer can beformed by a sputtering method. For example, in the case where anIn—Ga—Zn-based oxide is used for the semiconductor layer, thesemiconductor layer can be formed by a sputtering method using anIn—Ga—Zn-based oxide as a target. The other steps can be performed in amanner similar to that of the manufacturing method illustrated in FIG.17 to FIG. 19.

The above is the description of examples of a manufacturing method ofthe pixel and the like.

<Shape of Conductive Layer>

For a conductive layer that can be used as a wiring such as a gate lineor a source line, a low-resistance material such as metal or an alloy ispreferably used because it can reduce the wiring resistance. In the casewhere a display device having a large screen is manufactured, it is alsoeffective to increase the width of a wiring. However, such a conductivelayer does not transmit visible light, and in a transmissive liquidcrystal display device, a large width of a wiring itself and a reductionin aperture ratio owing to an increase in the number of wirings arecaused in some cases.

Thus, the shape of an end portion of the conductive layer is devised,whereby light from a backlight unit can be extracted efficiently.

FIG. 23(A) is a cross-sectional view of the conductive layer 33 forminga source line and the like and the vicinity thereof An end portion ofthe conductive layer 33 has an inverse tapered shape. The conductivelayer 33 can be the conductive layer 33 a, the conductive layer 33 b,and the conductive layer 33 c, for example. Alternatively, theconductive layer 33 can be the conductive layer 51, for example.

Here, a taper angle refers to an angle between a bottom surface (asurface in contact with a surface where a thin film is formed) and aside surface at an end portion of the thin film. The taper angle isgreater than 0° and less than 180°. A taper with an angle less than 90°is referred to as a forward taper whereas a taper with an angle greaterthan 90° is referred to as an inverse taper.

As illustrated in FIG. 23(A), when the conductive layer 33 has aninverse tapered shape, part of light 50 from the backlight unit isreflected by a side surface of the conductive layer 33 and reaches theliquid crystal 22. Consequently, the light extraction efficiency can beincreased compared with the case where a side surface of the conductivelayer 33 is perpendicular or has a forward tapered shape.

Here, the taper angle of the conductive layer 33 is preferably greaterthan 90° and less than 135°, further preferably greater than or equal to91° and less than or equal to 120°, still further preferably greaterthan or equal to 95° and less than or equal to 110°.

FIG. 23(B) illustrates an example of the case where the conductive layer31 forming a gate line and the like has an inverse tapered shape. Whenthe conductive layer 31 has an inverse tapered shape as well as theconductive layer 33, the light extraction efficiency can be increasedmore efficiently.

The above is the description of the shape of the conductive layer.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 2

In this embodiment, an example of a transistor that can be used for thedisplay device or the like described in the above embodiment isdescribed with reference to drawings.

Structure Example 1 of Transistor

Modification examples of the transistor illustrated in FIG. 9 to FIG. 12and the like are described below.

A transistor illustrated in FIG. 24(A) includes a semiconductor layer 37between the semiconductor layer 32 and the impurity semiconductor layers35.

The semiconductor layer 37 may be formed using the same semiconductorfilm as the semiconductor layer 32. The semiconductor layer 37 has afunction as an etching stopper for preventing the semiconductor layer 32from being removed by etching at the time of etching of the impuritysemiconductor layers 35. Although FIG. 24(A) illustrates an examplewhere the semiconductor layer 37 is divided into a right portion and aleft portion, part of the semiconductor layer 37 may cover a channelformation region of the semiconductor layer 32.

Furthermore, the semiconductor layer 37 may contain an impurity at aconcentration lower than that in the impurity semiconductor layer 35.Thus, the semiconductor layer 37 can function as an LDD (Lightly DopedDrain) region and can suppress the hot channel effect when thetransistor is driven.

In a transistor illustrated in FIG. 24(B), an insulating layer 84 isprovided over the channel formation region of the semiconductor layer32. The insulating layer 84 has a function as an etching stopper at thetime of etching of the impurity semiconductor layer 35.

A transistor illustrated in FIG. 24(C) includes a semiconductor layer 32p instead of the semiconductor layer 32. The semiconductor layer 32 pincludes a semiconductor film having high crystallinity. For example,the semiconductor layer 32 p contains a polycrystalline semiconductor ora single crystal semiconductor. Thus, a transistor having a highfield-effect mobility can be provided.

A transistor illustrated in FIG. 24(D) includes the semiconductor layer32 p in the channel formation region of the semiconductor layer 32. Forexample, the transistor illustrated in FIG. 24(D) can be formed byirradiating a semiconductor film to be the semiconductor layer 32 withlaser light or the like so that crystallization is caused locally. Thus,a transistor having a high field-effect mobility can be provided.

A transistor illustrated in FIG. 24(E) includes the semiconductor layer32 p having crystallinity in the channel formation region of thesemiconductor layer 32 of the transistor illustrated in FIG. 24(A).

A transistor illustrated in FIG. 24(F) includes the semiconductor layer32 p having crystallinity in the channel formation region of thesemiconductor layer 32 of the transistor illustrated in FIG. 24(B).

Structure Example 2 of Transistor

Modification examples of the transistor illustrated in FIG. 13 to FIG.16 and the like are described below.

As an example of a structure of a transistor, a transistor 200 a isdescribed with reference to FIGS. 25(A), (B), and (C). FIG. 25(A) is atop view of the transistor 200 a. FIG. 25(B) corresponds to across-sectional view of a cut plane taken along dashed-dotted line X1-X2in FIG. 25(A), and FIG. 25(C) corresponds to a cross-sectional view of acut plane taken along dashed-dotted line Y1-Y2 in FIG. 25(A). Note thatin FIG. 25(A), some components of the transistor 200 a (an insulatinglayer having a function as a gate insulating layer, and the like) areomitted to avoid complexity. Note that hereinafter, in some cases, thedirection of the dashed-dotted line X1-X2 is called the channel lengthdirection, and the direction of the dashed-dotted line Y1-Y2 is calledthe channel width direction. As in FIG. 25(A), in some cases, somecomponents are omitted from some top views of transistors describedbelow.

The transistor 200 a includes a conductive layer 221 over an insulatinglayer 224, an insulating layer 211 over the insulating layer 224 andover the conductive layer 221, a semiconductor layer 231 over theinsulating layer 211, a conductive layer 222 a over the semiconductorlayer 231 and over the insulating layer 211, a conductive layer 222 bover the semiconductor layer 231 and over the insulating layer 211, aninsulating layer 212 over the semiconductor layer 231, over theconductive layer 222 a, and over the conductive layer 222 b, and aconductive layer 223 over the insulating layer 212.

Note that the insulating layer 224 can be a substrate instead of aninsulating layer. In the case where the insulating layer 224 is asubstrate, the substrate can be a substrate that contains a materialsimilar to that of the substrate 14 described in Embodiment 1.

The conductive layer 221 and the conductive layer 223 can contain amaterial similar to that of the conductive layer 31 described inEmbodiment 1, for example. The insulating layer 211 can contain amaterial similar to that of the insulating layer 34 described inEmbodiment 1, for example. The conductive layer 222 a and the conductivelayer 222 b can contain a material similar to that of the conductivelayer 33 and the conductive layer 51 described in Embodiment 1, forexample. The insulating layer 212 can contain a material similar to thatof the insulating layer 82 described in Embodiment 1.

As the semiconductor layer 231, like the semiconductor layer 32described in Embodiment 1, a semiconductor layer containing a metaloxide can be used. In this embodiment, the case where the semiconductorlayer 231 is a semiconductor layer containing a metal oxide isdescribed.

The insulating layer 211 and the insulating layer 212 have an openingportion 235. The conductive layer 223 is electrically connected to theconductive layer 221 through the opening portion 235.

Here, the insulating layer 211 has a function as a first gate insulatinglayer of the transistor 200 a, and the insulating layer 212 has afunction as a second gate insulating layer of the transistor 200 a. Inthe transistor 200 a, the conductive layer 221 has a function as a firstgate, the conductive layer 222 a has a function as one of a source and adrain, and the conductive layer 222 b has a function as the other of thesource and the drain. In the transistor 200 a, the conductive layer 223has a function as a second gate.

Note that the transistor 200 a is what is called a channel-etchedtransistor, and has a dual-gate structure.

The transistor 200 a can have a structure in which the conductive layer223 is not provided. In that case, the transistor 200 a is what iscalled a channel-etched transistor, and has a bottom-gate structure.

As illustrated in FIGS. 25(B) and (C), the semiconductor layer 231 ispositioned to face the conductive layer 221 and the conductive layer223, and is sandwiched between two conductive layers having functions asthe gates. The length of the conductive layer 223 in the channel lengthdirection and the length of the conductive layer 223 in the channelwidth direction are longer than the length of the semiconductor layer231 in the channel length direction and the length of the semiconductorlayer 231 in the channel width direction, respectively, and the wholesemiconductor layer 231 is covered with the conductive layer 223 withthe insulating layer 212 therebetween.

In other words, the conductive layer 221 and the conductive layer 223are connected to each other in the opening portion 235 provided in theinsulating layer 211 and the insulating layer 212, and each have aregion located outside a side end portion of the semiconductor layer231.

With this structure, the semiconductor layer 231 included in thetransistor 200 a can be electrically surrounded by electric fields ofthe conductive layer 221 and the conductive layer 223. A devicestructure of a transistor in which electric fields of a first gate and asecond gate electrically surround a semiconductor layer where a channelformation region is formed, like in the transistor 200 a, can bereferred to as a surrounded channel (s-channel) structure.

Since the transistor 200 a has the s-channel structure, an electricfield for inducing a channel can be effectively applied to thesemiconductor layer 231 by the conductive layer 221 having a function asthe first gate; therefore, the current drive capability of thetransistor 200 a can be improved and high on-state currentcharacteristics can be obtained. Since the on-state current can beincreased, it is possible to reduce the size of the transistor 200 a. Inaddition, since the transistor 200 a has a structure in which thesemiconductor layer 231 is surrounded by the conductive layer 221 havinga function as the first gate and the conductive layer 223 having afunction as the second gate, the mechanical strength of the transistor200 a can be increased.

Since the transistor 200 a having the s-channel structure has a highfield-effect mobility and high driving capability, the use of thetransistor 200 a in a driver circuit, typically in a gate driver, allowsthe display device to have a narrow frame.

Next, as an example of a structure of a transistor, a transistor 200 bis described with reference to FIGS. 26(A), (B), and (C). FIG. 26(A) isa top view of the transistor 200 b. FIG. 26(B) corresponds to across-sectional view of a cut plane taken along dashed-dotted line X1-X2in FIG. 26(A), and FIG. 26(C) corresponds to a cross-sectional view of acut plane taken along dashed-dotted line Y1-Y2 in FIG. 26(A).

The transistor 200 b is different from the transistor 200 a in that thesemiconductor layer 231, the conductive layer 222 a, the conductivelayer 222 b, and the insulating layer 212 each have a stacked-layerstructure.

The insulating layer 212 includes an insulating layer 212 a over thesemiconductor layer 231, over the conductive layer 222 a, and over theconductive layer 222 b, and an insulating layer 212 b over theinsulating layer 212 a. The insulating layer 212 has a function ofsupplying oxygen to the semiconductor layer 231. That is, the insulatinglayer 212 contains oxygen. The insulating layer 212 a is an insulatinglayer that can transmit oxygen. Note that the insulating layer 212 aalso functions as a film that relieves damage to the semiconductor layer231 at the time of forming the insulating layer 212 b in a later step.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, preferably greater than or equal to 5 nm and less than or equal to50 nm can be used as the insulating layer 212 a.

In addition, it is preferable that the number of defects in theinsulating layer 212 a be small, and typically, it is preferable thatthe spin density corresponding to a signal that appears at g=2.001 dueto a dangling bond of silicon be lower than or equal to 3×10¹⁷ spins/cm³by ESR measurement. This is because if the density of defects containedin the insulating layer 212 a is high, oxygen is bonded to the defectsand the property of transmitting oxygen of the insulating layer 212 a islowered.

Note that in the insulating layer 212 a, in some cases, not all oxygenthat has entered the insulating layer 212 a from the outside moves tothe outside of the insulating layer 212 a but some oxygen remains in theinsulating layer 212 a. Furthermore, movement of oxygen occurs in theinsulating layer 212 a in some cases in such a manner that oxygen entersthe insulating layer 212 a and oxygen contained in the insulating layer212 a moves to the outside of the insulating layer 212 a. When an oxideinsulating layer that can transmit oxygen is formed as the insulatinglayer 212 a, oxygen released from the insulating layer 212 b providedover the insulating layer 212 a can be moved to the semiconductor layer231 through the insulating layer 212 a.

Note that as the insulating layer 212 a, an oxide insulating layerhaving a low density of states due to nitrogen oxide can be used. Notethat the density of states due to nitrogen oxide can be formed betweenthe energy of the valence band maximum of the metal oxide and the energyof the conduction band minimum of the metal oxide. A silicon oxynitridefilm that releases a small amount of nitrogen oxide, an aluminumoxynitride film that releases a small amount of nitrogen oxide, or thelike can be used as the above oxide insulating layer.

Note that a silicon oxynitride film that releases a small amount ofnitrogen oxide is a film that releases ammonia more than nitrogen oxidein thermal desorption spectroscopy (TDS) analysis; the amount ofreleased ammonia is typically greater than or equal to 1×10 ¹⁸/cm³ andless than or equal to 5×10¹⁹/cm³. Note that the amount of releasedammonia is the released amount by heat treatment with which the surfacetemperature of a film becomes higher than or equal to 50° C. and lowerthan or equal to 650° C., preferably higher than or equal to 50° C. andlower than or equal to 550° C.

Nitrogen oxide (NO_(x), x is greater than 0 and less than or equal to 2,preferably greater than or equal to 1 and less than or equal to 2),typically NO₂ or NO, forms levels in the insulating layer 212 a or thelike. The level is positioned in the energy gap of the semiconductorlayer 231. Therefore, when nitrogen oxide is diffused to the interfacebetween the insulating layer 212 a and the semiconductor layer 231, anelectron is in some cases trapped by the level on the insulating layer212 a side. As a result, the trapped electron remains in the vicinity ofthe interface between the insulating layer 212 a and the semiconductorlayer 231; thus, the threshold voltage of the transistor is shifted inthe positive direction.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Sincenitrogen oxide contained in the insulating layer 212 a reacts withammonia contained in the insulating layer 212 b in heat treatment,nitrogen oxide contained in the insulating layer 212 a is reduced.Therefore, an electron is hardly trapped at the interface between theinsulating layer 212 a and the semiconductor layer 231.

By using the above oxide insulating layer as the insulating layer 212 a,the shift in the threshold voltage of the transistor can be reduced,which leads to a smaller change in the electrical characteristics of thetransistor.

The concentration of nitrogen of the above oxide insulating layermeasured by SIMS is lower than or equal to 6×10 ²⁰ atoms/cm³.

The above oxide insulating layer is formed by a PECVD method usingsilane and dinitrogen monoxide at a substrate temperature higher than orequal to 220° C. and lower than or equal to 350° C., whereby a dense andhard film can be formed.

The insulating layer 212 b is an oxide insulating layer which containsoxygen at a higher proportion than the stoichiometric composition. Partof oxygen is released from the above oxide insulating layer by heating.Note that the above oxide insulating layer includes a region in whichthe amount of released oxygen in TDS is greater than or equal to1.0×10¹⁹ atoms/cm³, preferably greater than or equal to 3.0×10²⁰atoms/cm³. Note that the amount of released oxygen is the total amountin a temperature range of 50° C. to 650° C. or 50° C. to 550° C. in heattreatment in TDS. In addition, the amount of released oxygen is thetotal amount of released oxygen converted into oxygen atoms in TDS.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, preferably greater than or equal to 50 nm and less than or equal to400 nm can be used as the insulating layer 212 b.

It is preferable that the number of defects in the insulating layer 212b be small, and typically, it is preferable that the spin densitycorresponding to a signal that appears at g=2.001 due to a dangling bondof silicon be lower than 1.5×10¹⁸ spins/cm³, further preferably lowerthan or equal to 1×10¹⁸ spins/cm³ by ESR measurement. Note that theinsulating layer 212 b is provided more apart from the semiconductorlayer 231 than the insulating layer 212 a is, and thus may have higherdensity of defects than the insulating layer 212 a.

Furthermore, the insulating layer 212 a and the insulating layer 212 bcan be formed using insulating layers formed of the same kinds ofmaterials; thus, an interface between the insulating layer 212 a and theinsulating layer 212 b cannot be clearly observed in some cases. Thus,in this embodiment, the interface between the insulating layer 212 a andthe insulating layer 212 b is shown by a dashed line. Although atwo-layer structure of the insulating layer 212 a and the insulatinglayer 212 b is described in this embodiment, the present invention isnot limited to this; for example, a single-layer structure of theinsulating layer 212 a or a stacked-layer structure of three or morelayers may be employed.

The semiconductor layer 231 in the transistor 200 b includes asemiconductor layer 231_1 over the insulating layer 211 and asemiconductor layer 231_2 over the semiconductor layer 231_1. Thesemiconductor layer 231_1 and the semiconductor layer 231_2 contain thesame element. For example, it is preferable that the semiconductor layer231_1 and the semiconductor layer 231_2 each contain the same element asthe element in the above semiconductor layer 231.

Each of the semiconductor layer 231_1 and the semiconductor layer 231_2preferably includes a region where the atomic proportion of In is higherthan that of element M For example, the atomic ratio of In to M and Znin each of the semiconductor layer 231_1 and the semiconductor layer231_2 is preferably In:M:Zn=4:2:3 or in the neighborhood thereof. Here,neighborhood means that when In is 4, M ranges from 1.5 to 2.5 and Znranges from 2 to 4. Alternatively, the atomic ratio of In to M and Zn ineach of the semiconductor layer 231_1 and the semiconductor layer 231_2is preferably In:M:Zn=5:1:6 or in the neighborhood thereof. Thesemiconductor layer 231_1 and the semiconductor layer 231_2 havingsubstantially the same composition can be formed using the samesputtering target; thus, the manufacturing cost can be reduced. When thesame sputtering target is used, the semiconductor layer 231_1 and thesemiconductor layer 231_2 can be formed successively in the same chamberin a vacuum; therefore, entry of impurities into the interface betweenthe semiconductor layer 231_1 and the semiconductor layer 231_2 can besuppressed.

Here, the semiconductor layer 231_1 may include a region whosecrystallinity is lower than that of the semiconductor layer 231_2. Notethat the crystallinity of the semiconductor layer 231_1 and thesemiconductor layer 231_2 can be determined by analysis by X-raydiffraction (XRD) or analysis with a transmission electron microscope(TEM), for example.

The region with low crystallinity in the semiconductor layer 231_1serves as a diffusion path of excess oxygen, through which excess oxygencan also be diffused into the semiconductor layer 231_2 having highercrystallinity than the semiconductor layer 231_1. When a stacked-layerstructure including the semiconductor layers having different crystalstructures is employed and the region with low crystallinity is used asa diffusion path of excess oxygen as described above, a highly reliabletransistor can be provided.

The semiconductor layer 231_2 including a region with highercrystallinity than the semiconductor layer 231_1 can prevent impuritiesfrom entering the semiconductor layer 231. In particular, the increasedcrystallinity of the semiconductor layer 231_2 can suppress damage atthe time of forming the conductive layer 222 a and the conductive layer222 b. The surface of the semiconductor layer 231, i.e., the surface ofthe semiconductor layer 231_2 is exposed to an etchant or an etching gasat the time of forming the conductive layer 222 a and the conductivelayer 222 b. However, when the semiconductor layer 231_2 includes aregion with high crystallinity, the semiconductor layer 231_2 has higheretching resistance than the semiconductor layer 231_1 with lowcrystallinity. Thus, the semiconductor layer 231_2 has a function as anetching stopper.

By including a region with lower crystallinity than the semiconductorlayer 231_2, the semiconductor layer 231_1 sometimes has a high carrierdensity.

When the semiconductor layer 231_1 has a high carrier density, the Fermilevel is sometimes high relative to the conduction band of thesemiconductor layer 231_1. This lowers the conduction band minimum ofthe semiconductor layer 231_1, so that the energy difference between theconduction band minimum of the semiconductor layer 231_1 and the traplevel, which might be formed in a gate insulating layer (here, theinsulating layer 211), is increased in some cases. The increase of theenergy difference can reduce trap of charges in the gate insulatinglayer and reduce variation in the threshold voltage of the transistor,in some cases. In addition, when the semiconductor layer 231_1 has ahigh carrier density, the semiconductor layer 231 can have a highfield-effect mobility.

Although the semiconductor layer 231 in the transistor 200 b has astacked-layer structure including two layers in this example, thestructure is not limited thereto, and a structure in which three or morelayers are stacked may be employed.

The conductive layer 222 a included in the transistor 200 b includes aconductive layer 222 a_1, a conductive layer 222 a_2 over the conductivelayer 222 a_1, and a conductive layer 222 a_3 over the conductive layer222 a_2. The conductive layer 222 b included in the transistor 200 bincludes a conductive layer 222 b_1, a conductive layer 222 b_2 over theconductive layer 222 b_1, and a conductive layer 222 b_3 over theconductive layer 222 b_2.

For example, it is preferable that the conductive layer 222 a_1, theconductive layer 222 b_1, the conductive layer 222 a_3, and theconductive layer 222 b_3 contain one or more elements selected fromtitanium, tungsten, tantalum, molybdenum, indium, gallium, tin, andzinc. Furthermore, it is preferable that the conductive layer 222 a_2and the conductive layer 222 b_2 contain one or more elements selectedfrom copper, aluminum, and silver.

Specifically, an In—Sn oxide or an In—Zn oxide can be used for theconductive layer 222 a_1, the conductive layer 222 b_1, the conductivelayer 222 a_3, and the conductive layer 222 b_3 and copper can be usedfor the conductive layer 222 a_2 and the conductive layer 222 b_2.

An end portion of the conductive layer 222 a_1 has a region locatedoutside an end portion of the conductive layer 222 a_2, and theconductive layer 222 a_3 covers a top surface and a side surface of theconductive layer 222 a_2 and has a region that is in contact with theconductive layer 222 a_1. An end portion of the conductive layer 222 b_1has a region located outside an end portion of the conductive layer 222b_2, and the conductive layer 222 b_3 covers a top surface and a sidesurface of the conductive layer 222 b_2 and has a region that is incontact with the conductive layer 222 b_1.

The above structure is preferred because the wiring resistance of theconductive layer 222 a and the conductive layer 222 b can be reduced anddiffusion of copper to the semiconductor layer 231 can be inhibited.

Next, as an example of a structure of a transistor, a transistor 200 cis described with reference to FIGS. 27(A), (B), and (C). FIG. 27(A) isa top view of the transistor 200 c. FIG. 27(B) corresponds to across-sectional view of a cut plane taken along dashed-dotted line X1-X2in FIG. 27(A), and FIG. 27(C) corresponds to a cross-sectional view of acut plane taken along dashed-dotted line Y1-Y2 in FIG. 27(A).

The transistor 200 c includes the conductive layer 221 over theinsulating layer 224, the insulating layer 211 over the conductive layer221 and over the insulating layer 224, the semiconductor layer 231 overthe insulating layer 211, an insulating layer 216 over the semiconductorlayer 231 and over the insulating layer 211, the conductive layer 222 aover the semiconductor layer 231 and over the insulating layer 216, theconductive layer 222 b over the semiconductor layer 231 and over theinsulating layer 216, the insulating layer 212 over the insulating layer216, the conductive layer 222 a, and the conductive layer 222 b, and theconductive layer 223 over the insulating layer 212.

The insulating layer 211, the insulating layer 216, and the insulatinglayer 212 have the opening portion 235. The conductive layer 221 havinga function as a first gate of the transistor 200 c is electricallyconnected to the conductive layer 223 having a function as a second gateof the transistor 200 c through the opening portion 235. The insulatinglayer 216 has an opening portion 238 a and an opening portion 238 b. Theconductive layer 222 a having a function as one of a source and a drainof the transistor 200 c is electrically connected to the semiconductorlayer 231 through the opening portion 238 a. The conductive layer 222 bhaving a function as the other of the source and the drain of thetransistor 200 c is electrically connected to the semiconductor layer231 through the opening portion 238 b.

The insulating layer 216 has a function as a channel protective layer ofthe transistor 200 c. Without the insulating layer 216, a channelformation region of the semiconductor layer 231 might be damaged in somecases when the conductive layer 222 a and the conductive layer 222 b areformed by an etching method or the like. This might make the electricalcharacteristics of the transistor unstable in some cases. The damage tothe channel formation region of the semiconductor layer 231 can beprevented when the insulating layer 216 is formed, a conductive layer isformed after the opening portion 238 a and the opening portion 238 b areprovided, and the conductive layer is processed by an etching method orthe like to form the conductive layer 222 a and the conductive layer 222b. Accordingly, the electrical characteristics of the transistor can bestabilized and a highly reliable transistor can be provided.

The insulating layer 216 can include a material similar to that of theinsulating layer 212, for example.

The insulating layer 216 preferably includes an excess oxygen region,and when the insulating layer 216 includes an excess oxygen region,oxygen can be supplied to the channel formation region of thesemiconductor layer 231. As a result, oxygen vacancies formed in thechannel formation region can be filled with excess oxygen, which canprovide a highly reliable display device.

After the opening portion 238 a and the opening portion 238 b areformed, an impurity element is preferably added to the semiconductorlayer 231. Specifically, an element that forms an oxygen vacancy or anelement that is bonded to an oxygen vacancy is preferably added. Thiscan increase the conductivity of a region of the semiconductor layer 231which overlaps with the conductive layer 222 a (one of a source regionand a drain region) and a region of the semiconductor layer 231 whichoverlaps with the conductive layer 222 b (the other of the source regionand the drain region), the detail of which is to be described later.Accordingly, the current drive capability of the transistor 200 c isimproved, so that high on-state current characteristics can be obtained.

Note that the transistor 200 c is what is called a channel-protectivetransistor, and has a dual-gate structure.

Like the transistor 200 a and the transistor 200 b, the transistor 200 chas the s-channel structure. With this structure, the semiconductorlayer 231 included in the transistor 200 c can be electricallysurrounded by electric fields of the conductive layer 221 and theconductive layer 223.

Since the transistor 200 c has the s-channel structure, an electricfield for inducing a channel can be effectively applied to thesemiconductor layer 231 by the conductive layer 221 or the conductivelayer 223. Thus, the current drive capability of the transistor 200 ccan be improved and high on-state current characteristics can beobtained. As a result of the high on-state current, it is possible toreduce the size of the transistor 200 c. Furthermore, since thetransistor 200 c has a structure in which the semiconductor layer 231 issurrounded by the conductive layer 221 and the conductive layer 223, themechanical strength of the transistor 200 c can be increased.

Note that the transistor 200 c can have a structure in which theconductive layer 223 is not provided. In that case, the transistor 200 cis what is called a channel-protective transistor, and has a bottom-gatestructure.

Next, an example of a structure of a transistor is described withreference to FIGS. 28(A), (B), (C), and (D).

FIGS. 28(A) and (B) are cross-sectional views of a transistor 200 d andFIGS. 28(C) and (D) are cross-sectional views of a transistor 200 e.Note that the transistor 200 d is a modification example of theabove-described transistor 200 b and the transistor 200 e is amodification example of the above-described transistor 200 c. Thus, inFIGS. 28(A), (B), (C), and (D), common reference numerals are used forthe components having functions similar to those in the transistor 200 band the transistor 200 c, and a detailed description is omitted.

FIG. 28(A) is a cross-sectional view of the transistor 200 d in thechannel length direction, and FIG. 28(B) is a cross-sectional view ofthe transistor 200 d in the channel width direction. FIG. 28(C) is across-sectional view of the transistor 200 e in the channel lengthdirection, and FIG. 28(D) is a cross-sectional view of the transistor200 e in the channel width direction.

Compared with the transistor 200 b, the transistor 200 d illustrated inFIGS. 28(A) and (B) is not provided with the conductive layer 223 andthe opening portion 235. The transistor 200 d is different from thetransistor 200 b in the structures of the insulating layer 212, theconductive layer 222 a, and the conductive layer 222 b.

In the transistor 200 d, the insulating layer 212 includes an insulatinglayer 212 c and an insulating layer 212 d over the insulating layer 212c. The insulating layer 212 c has a function of supplying oxygen to thesemiconductor layer 231 and a function of preventing entry of impurities(typically, water, hydrogen, and the like). As the insulating layer 212c, an aluminum oxide film, an aluminum oxynitride film, or an aluminumnitride oxide film can be used. In particular, the insulating layer 212c is preferably an aluminum oxide film formed by a reactive sputteringmethod. As an example of a method of forming an aluminum oxide film by areactive sputtering method, the following method can be given.

First, a mixed gas of an inert gas (typically, an Ar gas) and an oxygengas is introduced into a sputtering chamber. Subsequently, a voltage isapplied to an aluminum target provided in the sputtering chamber,whereby the aluminum oxide film can be deposited. As an electric powersource for applying a voltage to the aluminum target, a DC power source,an AC power source, or an RF power source can be given. The DC powersource is particularly preferable because the productivity can beimproved.

The insulating layer 212 d has a function of preventing entry ofimpurities (typically, water, hydrogen, and the like). As the insulatinglayer 212 d, a silicon nitride film, a silicon nitride oxide film, or asilicon oxynitride film can be used. In particular, a silicon nitridefilm formed by a PECVD method is preferably used as the insulating layer212 d. The silicon nitride film formed by a PECVD method is preferablebecause the film is likely to have a high film density. Note that thehydrogen concentration in the silicon nitride film formed by a PECVDmethod is high in some cases.

Since the insulating layer 212 c is provided below the insulating layer212 d in the transistor 200 d, hydrogen contained in the insulatinglayer 212 d does not or is less likely to diffuse into the semiconductorlayer 231 side.

Note that the transistor 200 d is a transistor having a single-gatestructure, unlike the transistor 200 b. The use of a transistor having asingle-gate structure can reduce the number of masks, leading toincreased productivity.

The transistor 200 e illustrated in FIGS. 28(C) and (D) is differentfrom the transistor 200 c in the structures of the insulating layer 216and the insulating layer 212. Specifically, the transistor 200 eincludes an insulating layer 216 a instead of the insulating layer 216,and the insulating layer 212 d instead of the insulating layer 212. Inaddition, in the transistor 200 e, the semiconductor layer 231 includesthe semiconductor layer 231_1 and the semiconductor layer 231_2.

The insulating layer 216 a has a function similar to that of theinsulating layer 212 c.

The structure of the transistor 200 d or the transistor 200 e can beformed using the existing production line without high capitalinvestment. For example, a production line for an oxide semiconductorcan be simply substituted for a production line for hydrogenatedamorphous silicon.

Next, as an example of a structure of a transistor, a transistor 200 fis described with reference to FIGS. 29(A), (B), and (C). FIG. 29(A) isa top view of the transistor 200 f FIG. 29(B) corresponds to across-sectional view of a cut plane taken along dashed-dotted line X1-X2in FIG. 29(A), and FIG. 29(C) corresponds to a cross-sectional view of acut plane taken along dashed-dotted line Y1-Y2 in FIG. 29(A).

The transistor 200 f illustrated in FIGS. 29(A), (B), and (C) includesthe conductive layer 221 over the insulating layer 224, the insulatinglayer 211 over the conductive layer 221 and over the insulating layer224, the semiconductor layer 231 over the insulating layer 211, theinsulating layer 212 over the semiconductor layer 231, the conductivelayer 223 over the insulating layer 212, and an insulating layer 215over the insulating layer 211, over the semiconductor layer 231, andover the conductive layer 223. Note that the semiconductor layer 231includes a channel formation region 231 i overlapping with theconductive layer 223, a source region 231 s in contact with theinsulating layer 215, and a drain region 231 d in contact with theinsulating layer 215.

The insulating layer 215 contains nitrogen or hydrogen. The insulatinglayer 215 is in contact with the source region 231 s and the drainregion 231 d, so that nitrogen or hydrogen in the insulating layer 215is added to the source region 231 s and the drain region 231 d. Thesource region 231 s and the drain region 231 d each have a high carrierdensity when nitrogen or hydrogen is added thereto.

The transistor 200 f may include the conductive layer 222 a electricallyconnected to the source region 231 s through an opening portion 236 aprovided in the insulating layer 215. The transistor 200 f may furtherinclude the conductive layer 222 b electrically connected to the drainregion 231 d through an opening portion 236 b provided in the insulatinglayer 215.

The insulating layer 211 has a function as a first gate insulatinglayer, and the insulating layer 212 has a function as a second gateinsulating layer. The insulating layer 215 has a function as aprotective insulating layer.

The insulating layer 212 includes an excess oxygen region. When theinsulating layer 212 includes an excess oxygen region, excess oxygen canbe supplied to the channel formation region 231 i included in thesemiconductor layer 231. As a result, oxygen vacancies that might beformed in the channel formation region 231 i can be filled with excessoxygen, which can provide a highly reliable display device.

Note that to supply excess oxygen to the semiconductor layer 231, excessoxygen may be supplied to the insulating layer 211 formed below thesemiconductor layer 231. However, in that case, excess oxygen containedin the insulating layer 211 might also be supplied to the source region231 s and the drain region 231 d included in the semiconductor layer231. When excess oxygen is supplied to the source region 231 s and thedrain region 231 d, the resistance of the source region 231 s and thedrain region 231 d might be increased in some cases.

By contrast, in the structure in which the insulating layer 212 formedover the semiconductor layer 231 contains excess oxygen, excess oxygencan be selectively supplied only to the channel formation region 231 i.Alternatively, the carrier densities of the source region 231 s and thedrain region 231 d are selectively increased after excess oxygen issupplied to the channel formation region 231 i, the source region 231 s,and the drain region 231 d, in which case an increase in the resistanceof the source region 231 s and the drain region 231 d can be prevented.

Furthermore, each of the source region 231 s and the drain region 231 dincluded in the semiconductor layer 231 preferably contains an elementthat forms an oxygen vacancy or an element that is bonded to an oxygenvacancy. Typical examples of the element that forms an oxygen vacancy orthe element that is bonded to an oxygen vacancy include hydrogen, boron,carbon, nitrogen, fluorine, phosphorus, sulfur, chlorine, titanium, anda rare gas element. Typical examples of the rare gas element includehelium, neon, argon, krypton, and xenon. In the case where one or moreof the elements that form oxygen vacancies or the elements that arebonded to oxygen vacancies are contained in the insulating layer 215,the one or more of the elements are diffused from the insulating layer215 to the source region 231 s and the drain region 231 d, and/or addedto the source region 231 s and the drain region 231 d by impurityaddition treatment.

When an impurity element is added to the metal oxide, a bond between ametal element and oxygen in the metal oxide is cut, so that an oxygenvacancy is formed. Alternatively, when an impurity element is added tothe metal oxide, oxygen bonded to a metal element in the metal oxide isbonded to the impurity element and the oxygen is released from the metalelement, so that an oxygen vacancy is formed. As a result, the carrierdensity of the metal oxide is increased and thus the conductivitythereof becomes higher.

The conductive layer 221 has a function as a first gate, the conductivelayer 223 has a function as a second gate, the conductive layer 222 ahas a function as a source, and the conductive layer 222 b has afunction as a drain.

As illustrated in FIG. 29(C), an opening portion 237 is provided in theinsulating layer 211 and the insulating layer 212. The conductive layer221 is electrically connected to the conductive layer 223 through theopening portion 237. Thus, the conductive layer 221 and the conductivelayer 223 are supplied with the same potential. Note that differentpotentials may be supplied to the conductive layer 221 and theconductive layer 223 without providing the opening portion 237.Alternatively, the conductive layer 221 may be used as a light-blockingfilm without providing the opening portion 237. For example, lightemitted to the channel formation region 231 i from the bottom can besuppressed when the conductive layer 221 is formed with a light-blockingmaterial.

As illustrated in FIGS. 29(B) and (C), while facing the conductive layer221 having a function as the first gate and the conductive layer 223having a function as the second gate, the semiconductor layer 231 ispositioned between the two conductive layers having functions as thegates.

Like the transistor 200 a, the transistor 200 b, and the transistor 200c, the transistor 200 f has the s-channel structure. With thisstructure, the semiconductor layer 231 included in the transistor 200 fcan be electrically surrounded by electric fields of the conductivelayer 221 having a function as the first gate and the conductive layer223 having a function as the second gate.

Since the transistor 200 f has the s-channel structure, an electricfield for inducing a channel can be effectively applied to thesemiconductor layer 231 by the conductive layer 221 or the conductivelayer 223. Thus, the current drive capability of the transistor 200 fcan be improved and high on-state current characteristics can beobtained. As a result of the high on-state current, it is possible toreduce the size of the transistor 200 f. Furthermore, since thetransistor 200 f has a structure in which the semiconductor layer 231 issurrounded by the conductive layer 221 and the conductive layer 223, themechanical strength of the transistor 200 f can be increased.

The transistor 200 f may be called a TGSA (Top Gate Self Aligned) FETfrom the position of the conductive layer 223 relative to thesemiconductor layer 231 or the formation method of the conductive layer223.

The semiconductor layer 231 in the transistor 200 f may have astacked-layer structure including two or more layers, as in thetransistor 200 b.

Although the insulating layer 212 is provided only in a portionoverlapping with the conductive layer 223 in the transistor 200 f, thestructure is not limited thereto, and the structure in which theinsulating layer 212 covers the semiconductor layer 231 can be employed.Alternatively, a structure in which the conductive layer 221 is notprovided can be employed.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments and the like asappropriate.

Embodiment 3

In this embodiment, examples of a method of crystallization forpolycrystalline silicon which can be used for a semiconductor layer of atransistor and a laser crystallization apparatus are described.

To form a polycrystalline silicon layer having favorable crystallinity,it is preferable that an amorphous silicon layer be provided over asubstrate and the amorphous silicon layer be crystallized by laserirradiation. For example, a linear beam is used as laser light and thesubstrate is moved while the amorphous silicon layer is irradiated withthe linear beam, so that a polycrystalline silicon layer can be formedin a desired region over the substrate.

The method using a linear beam is relatively favorable in throughput. Onthe other hand, it is a method in which laser light is moved relative toa region and is emitted a plurality of times; thus, variation incrystallinity tends to be produced owing to a change in the output oflaser light and a change in the beam profile caused by the outputchange. For example, when a semiconductor layer crystallized by thismethod is used for a transistor included in a pixel of a display device,a random stripe pattern caused by variation in crystallinity might beobserved when an image is displayed.

The length of the linear beam is ideally greater than or equal to thelength of a side of the substrate; however, the length of the linearbeam is limited by an output of a laser and the structure of an opticalsystem. Thus, it is practical to irradiate a large substrate with thelaser light by turning back the laser light in a substrate plane.Consequently, there is a region irradiated with the laser light aplurality of times. Since the crystallinity of such a region is likelyto be different from that of the other region, display unevenness issometimes caused in the region.

To avoid such a problem as mentioned above, an amorphous silicon layerformed over a substrate may be crystallized by local laser irradiation.Local laser irradiation easily forms a polycrystalline silicon layerwith small variation in crystallinity.

FIG. 30(A) is a diagram illustrating a method of locally irradiating anamorphous silicon layer formed over a substrate with laser light.

Laser light 826 emitted from an optical system unit 821 is reflected bya mirror 822 and enters a microlens array 823. The microlens array 823collects the laser light 826 to form a plurality of laser beams 827.

A substrate 830 over which an amorphous silicon layer 840 is formed isfixed to a stage 815. The amorphous silicon layer 840 is irradiated withthe plurality of laser beams 827, so that a plurality of polycrystallinesilicon layers 841 can be formed at the same time.

Microlenses of the microlens array 823 are preferably provided to matchwith a pixel pitch of the display device. Alternatively, they may beprovided at intervals of an integral multiple of the pixel pitch. Ineither of the cases, polycrystalline silicon layers can be formed inregions corresponding to all pixels by repeating the laser irradiationand the movement of the stage 815 in the X direction or the Y direction.

For example, when the microlens array 823 includes M rows and N columns(M and N are natural numbers) of microlenses arranged with a pixelpitch, laser irradiation is performed at a predetermined start positionfirst, so that M rows and N columns of polycrystalline silicon layers841 can be formed. Then, the stage 815 is moved by N columns in the rowdirection, laser irradiation is performed, and M rows and N columns ofpolycrystalline silicon layers 841 are further formed; consequently, Mrows and 2N columns of polycrystalline silicon layers 841 can be formed.By repeating the steps, a plurality of polycrystalline silicon layers841 can be formed in desired regions. In the case where laserirradiation is performed by turning back the laser light, laserirradiation is performed after movement in the row direction by adistance of N columns, and laser irradiation is repeated after movementin the column direction by a distance of M rows.

Note that even in the case of a method in which laser irradiation isperformed while the stage 815 is moved in one direction, polycrystallinesilicon layers can be formed with a pixel pitch by adjusting theoscillation frequency of the laser light and the moving speed of thestage 815 properly.

The size of the laser beam 827 can be approximately an area in which thewhole semiconductor layer of a transistor is included, for example.Alternatively, the size can be approximately an area in which the wholechannel formation region of a transistor is included. Furtheralternatively, the size can be approximately an area in which part of achannel formation region of a transistor is included. The size can beselected from them depending on required electrical characteristics of atransistor.

Note that in the case of a display device including a plurality oftransistors in a pixel, the size of the laser beam 827 can beapproximately an area in which the whole semiconductor layer of eachtransistor in a pixel is included. Alternatively, the size of the laserbeam 827 may be approximately an area in which the whole semiconductorlayers of transistors in a plurality of pixels are included.

As illustrated in FIG. 31(A), a mask 824 may be provided between themirror 822 and the microlens array 823. The mask 824 is provided with aplurality of opening portions corresponding to respective microlenses.The shape of the opening portion can be projected in the shape of thelaser beam 827; as illustrated in FIG. 31(A), the laser beam 827 havinga circular shape can be obtained in the case where the mask 824 includescircular opening portions. The laser beam 827 having a rectangular shapecan be obtained in the case where the mask 824 includes rectangularopening portions. The mask 824 is effective in the case where only achannel formation region of a transistor is crystallized, for example.Note that the mask 824 may be provided between the optical system unit821 and the mirror 822 as illustrated in FIG. 31(B).

FIG. 30(B) is a perspective view illustrating a main structure of alaser crystallization apparatus which can be used in the above locallaser irradiation step. The laser crystallization apparatus includes amoving mechanism 812, a moving mechanism 813, and the stage 815 whichare components of an X-Y stage. A laser 820, the optical system unit821, the mirror 822, and the microlens array 823 to shape the laser beam827 are further included.

The moving mechanism 812 and the moving mechanism 813 each have afunction of performing reciprocating linear motion in the horizontaldirection. As a mechanism for powering the moving mechanism 812 and themoving mechanism 813, a ball screw mechanism 816 driven by a motor canbe used, for example. The moving directions of the moving mechanism 812and the moving mechanism 813 cross orthogonally; thus, the stage 815fixed to the moving mechanism 813 can be moved in the X direction and inthe Y direction freely.

The stage 815 includes a fixing mechanism such as a vacuum suctionmechanism and can fix the substrate 830 or the like. Furthermore, thestage 815 may include a heating mechanism as needed. Although notillustrated, the stage 815 includes a pusher pin and a vertical movingmechanism thereof, and the substrate 830 or the like can be moved up anddown when the substrate 830 or the like is transferred.

The laser 820 is preferably a pulsed laser, but may be a CW laser aslong as light with a wavelength and intensity suitable for the purposeof processing can be output. Typically, an excimer laser that can emitultraviolet light with a wavelength of 351 nm to 353 nm (XeF), awavelength of 308 nm (XeCl), or the like can be used. Alternatively, asecond harmonic wavelength (515 nm, 532 nm, or the like) or a thirdharmonic wavelength (343 nm, 355 nm, or the like) of a solid-state laser(a YAG laser, a fiber laser, or the like) may be used. A plurality oflasers 820 may be provided.

The optical system unit 821 includes a mirror, a beam expander, a beamhomogenizer, and the like, for example, and can expand laser light 825while homogenizing the energy in-plane distribution of the laser light825 output from the laser 820.

As the mirror 822, a dielectric multilayer mirror can be used, forexample, and is provided so that the incident angle of the laser lightis substantially 45°. The microlens array 823 can have a shape such thata plurality of convex lenses are provided on the top surface or on thetop and bottom surfaces of a quartz board, for example.

With use of the above-described laser crystallization apparatus,polycrystalline silicon layers with small variation in crystallinity canbe formed.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 4

The composition of a CAC-OS applicable to a transistor disclosed in oneembodiment of the present invention is described below.

A CAC-OS refers to one composition of a material in which elementsconstituting a metal oxide are unevenly distributed with a size greaterthan or equal to 0.5 nm and less than or equal to 10 nm, preferablygreater than or equal to 1 nm and less than or equal to 2 nm, or asimilar size, for example. Note that a state in which one or more metalelements are unevenly distributed and regions including the metalelement(s) are mixed with a size greater than or equal to 0.5 nm andless than or equal to 10 nm, preferably greater than or equal to 1 nmand less than or equal to 2 nm, or a similar size in a metal oxide ishereinafter referred to as a mosaic pattern or a patch-like pattern.

Note that a metal oxide preferably contains at least indium. It isparticularly preferable that indium and zinc are contained. Moreover, inaddition to these, one kind or a plurality of kinds selected fromaluminum, gallium, yttrium, copper, vanadium, beryllium, boron, silicon,titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum,cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the likemay be contained.

For instance, a CAC-OS in an In—Ga—Zn oxide (an In—Ga—Zn oxide in theCAC-OS may be particularly referred to as CAC-IGZO) has a composition inwhich materials are separated into indium oxide (hereinafter InO_(X1)(X1is a real number greater than 0)) or indium zinc oxide (hereinafterIn_(X2)Zn_(Y2)O_(Z2) (X2, Y2, and Z2 are real numbers greater than 0))and gallium oxide (hereinafter GaO_(X3) (X3 is a real number greaterthan 0)) or gallium zinc oxide (hereinafter Ga_(X4)Zn_(Y4)O_(Z4) (X4,Y4, and Z4 are real numbers greater than 0)), for example, so that amosaic pattern is formed, and mosaic-like InO_(X1) orIn_(X2)Zn_(Y2)O_(Z2) is evenly distributed in the film (which ishereinafter also referred to as cloud-like).

That is, the CAC-OS is a composite metal oxide having a composition inwhich a region including GaO_(X3) as a main component and a regionincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component aremixed. Note that in this specification, for example, when the atomicratio of In to an element M in a first region is larger than the atomicratio of In to the element M in a second region, the first region isregarded as having a higher In concentration than the second region.

Note that IGZO is a commonly known name and sometimes refers to onecompound formed of In, Ga, Zn, and O. A typical example is a crystallinecompound represented by InGaO₃(ZnO)_(m1) (m1 is a natural number) orIn_((1+x0))Ga_((1−x0))O₃(ZnO)_(m0) (−1≤x0≤1; m0 is a given number).

The above crystalline compound has a single crystal structure, apolycrystalline structure, or a CAAC structure. Note that the CAACstructure is a crystal structure in which a plurality of IGZOnanocrystals have c-axis alignment and are connected in the a-b planedirection without alignment.

On the other hand, the CAC-OS relates to the material composition of ametal oxide. The CAC-OS refers to a composition in which, in thematerial composition containing In, Ga, Zn, and O, some regions thatinclude Ga as a main component and are observed as nanoparticles andsome regions that include In as a main component and are observed asnanoparticles are randomly dispersed in a mosaic pattern. Therefore, thecrystal structure is a secondary element for the CAC-OS.

Note that the CAC-OS is regarded as not including a stacked-layerstructure of two or more kinds of films with different compositions. Forexample, a two-layer structure of a film including In as a maincomponent and a film including Ga as a main component is not included.

Note that a clear boundary cannot sometimes be observed between theregion including GaO_(X3) as a main component and the region includingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component.

Note that in the case where one kind or a plurality of kinds selectedfrom aluminum, yttrium, copper, vanadium, beryllium, boron, silicon,titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum,cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the likeare contained instead of gallium, the CAC-OS refers to a composition inwhich some regions that include the metal element(s) as a main componentand are observed as nanoparticles and some regions that include In as amain component and are observed as nanoparticles are randomly dispersedin a mosaic pattern.

The CAC-OS can be formed by a sputtering method under a condition wherea substrate is not heated intentionally, for example. Moreover, in thecase of forming the CAC-OS by a sputtering method, any one or moreselected from an inert gas (typically, argon), an oxygen gas, and anitrogen gas are used as a deposition gas. Furthermore, the ratio of theflow rate of an oxygen gas to the total flow rate of the deposition gasat the time of deposition is preferably as low as possible, and forexample, the ratio of the flow rate of the oxygen gas is preferablyhigher than or equal to 0% and lower than 30%, further preferably higherthan or equal to 0% and lower than or equal to 10%.

The CAC-OS is characterized in that no clear peak is observed inmeasurement using θ/2θ scan by an Out-of-plane method, which is one ofX-ray diffraction (XRD) measurement methods. That is, it is found fromthe X-ray diffraction that no alignment in the a-b plane direction andthe c-axis direction is observed in a measured region.

In addition, in an electron diffraction pattern of the CAC-OS which isobtained by irradiation with an electron beam with a probe diameter of 1nm (also referred to as a nanobeam electron beam), a ring-likehigh-luminance region and a plurality of bright spots in the ring regionare observed. It is therefore found from the electron diffractionpattern that the crystal structure of the CAC-OS includes an nc(nano-crystal) structure with no alignment in the plan-view directionand the cross-sectional direction.

Moreover, for example, it can be confirmed by EDX mapping obtained usingenergy dispersive X-ray spectroscopy (EDX) that the CAC-OS in theIn—Ga—Zn oxide has a composition in which regions including GaO_(X3) asa main component and regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1)as a main component are unevenly distributed and mixed.

The CAC-OS has a composition different from that of an IGZO compound inwhich the metal elements are evenly distributed, and has characteristicsdifferent from those of the IGZO compound. That is, the CAC-OS has acomposition in which regions including GaO_(X3) or the like as a maincomponent and regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as amain component are phase-separated from each other and form a mosaicpattern.

Here, a region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent is a region whose conductivity is higher than that of a regionincluding GaO_(X3) or the like as a main component. In other words, whencarriers flow through the regions including In_(X2)Zn_(Y2)O_(Z2) orInO_(X1) as a main component, the conductivity of a metal oxide isexhibited. Accordingly, when the regions including In_(X2)Zn_(Y2)O_(Z2)or InO_(X1) as a main component are distributed in a metal oxide like acloud, high field-effect mobility (μ) can be achieved.

In contrast, a region including GaO_(X3) or the like as a main componentis a region whose insulating property is higher than that of a regionincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component. In otherwords, when regions including GaO_(X3) or the like as a main componentare distributed in an oxide semiconductor, leakage current can besuppressed and favorable switching operation can be achieved.

Accordingly, when the CAC-OS is used for a semiconductor element, theinsulating property derived from GaO_(X3) or the like and theconductivity derived from In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) complementeach other, whereby a high on-state current (I_(on)) and highfield-effect mobility (μ) can be achieved.

Moreover, a semiconductor element using the CAC-OS has high reliability.Thus, the CAC-OS is most suitable for a variety of semiconductor devicessuch as displays.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 5

In this embodiment, another structure example of the display devicedescribed in the above embodiment will be described.

FIG. 32 illustrates a structure example of the display device 10. Thedisplay device 10 includes the display portion 17 provided over thesubstrate 14. The display portion 17 includes the plurality of pixels 11connected to the wirings GL and the wirings SL.

Furthermore, the display device 10 is provided with a plurality of TAB(Tape Automated Bonding) tapes 121 a and a plurality of TAB tapes 121 b.The TAB tapes 121 a and the TAB tapes 121 b are provided to face eachother with the display portion 17 therebetween. Integrated circuits inwhich the gate drivers 12 a and the like are formed are mounted on theTAB tapes 121 a, and integrated circuits in which the gate drivers 12 band the like are formed are mounted on the TAB tapes 121 b. The gatedrivers 12 a and the gate drivers 12 b are connected to the plurality ofwirings GL and have a function of supplying selection signals to thewirings GL.

In addition, the display device 10 is provided with a plurality ofprinted boards 131 a and a plurality of TAB tapes 132 a, and providedwith a plurality of printed boards 131 b and a plurality of TAB tapes132 b. The printed boards 131 a and the TAB tapes 132 a are provided toface the printed boards 131 b and the TAB tapes 132 b with the displayportion 17 therebetween.

The printed boards 131 a are each connected to the plurality of TABtapes 132 a and have a function of distributing external input signalsto the TAB tapes 132 a. The printed boards 131 b are each connected tothe plurality of TAB tapes 132 b and have a function of distributingexternal input signals to the TAB tapes 132 b. Integrated circuits inwhich the source drivers 13 a and the like are formed are mounted on theTAB tapes 132 a, and integrated circuits in which the source drivers 13b and the like are formed are mounted on the TAB tapes 132 b. The sourcedrivers 13 a and the source drivers 13 b are connected to the pluralityof wirings SL and have a function of supplying signals to the wiringsSL.

In the case where a large-screen display panel which is compatible with2K, 4K, or 8K broadcasting or the like is manufactured, the plurality ofprinted boards 131 a and the plurality of printed boards 131 b arepreferably provided as illustrated in FIG. 32. Accordingly, input ofimage data to the display device 10 is facilitated.

Note that the gate drivers 12 a, the gate drivers 12 b, the sourcedrivers 13 a, and the source drivers 13 b can also be provided over thesubstrate 14 by a COG (Chip On Glass) method, a COF (Chip On Film)method, or the like.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 6

In this embodiment, electronic appliances of embodiments of the presentinvention will be described with reference to drawings.

Each of the electronic devices described below is provided with thedisplay device of one embodiment of the present invention in a displayportion. Thus, the electronic appliances achieve high resolution. Inaddition, the electronic appliances can achieve both high resolution anda large screen.

The display portion of the electronic appliance of one embodiment of thepresent invention can display, for example, an image with a resolutionof full high definition, 4K2K, 8K4K, 16K8K, or more. As a screen size ofthe display portion, the diagonal size can be greater than or equal to20 inches, greater than or equal to 30 inches, greater than or equal to50 inches, greater than or equal to 60 inches, or greater than or equalto 70 inches.

Examples of the electronic appliances include electronic appliances witha relatively large screen, such as a television device, a desktop orlaptop personal computer, a monitor of a computer or the like, a digitalsignage, and a large game machine such as a pachinko machine; a digitalcamera; a digital video camera; a digital photo frame; a mobile phone; aportable game console; a portable information terminal; and an audioreproducing device.

The electronic appliance or a lighting device of one embodiment of thepresent invention can be incorporated along a curved surface of aninside wall or an outside wall of a house or a building or a curvedsurface of an interior or exterior of an automobile.

The electronic appliance of one embodiment of the present invention mayinclude an antenna. When a signal is received by the antenna, an image,information, or the like can be displayed on a display portion. When theelectronic appliance includes the antenna and a secondary battery, theantenna may be used for contactless power transmission.

The electronic appliance of one embodiment of the present invention mayinclude a sensor (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, electriccurrent, voltage, electric power, radiation, flow rate, humidity,gradient, oscillation, odor, or infrared rays).

The electronic appliance of one embodiment of the present invention canhave a variety of functions. For example, the electronic appliance canhave a function of displaying a variety of information (e.g., a stillimage, a moving image, and a text image) on the display portion, a touchpanel function, a function of displaying a calendar, date, time, and thelike, a function of executing a variety of software (programs), awireless communication function, and a function of reading out a programor data stored in a recording medium.

FIG. 33(A) illustrates an example of a television device. In atelevision device 7100, a display portion 7000 is incorporated in ahousing 7101. Here, a structure in which the housing 7101 is supportedby a stand 7103 is shown.

The display device of one embodiment of the present invention can beused in the display portion 7000. In this way, the television device7100 can display a high-resolution image. The television device 7100 candisplay a high-resolution image on a large screen.

The television device 7100 illustrated in FIG. 33(A) can be operatedwith an operation switch provided in the housing 7101 or a separateremote controller 7111. Furthermore, the display portion 7000 mayinclude a touch sensor and the television device 7100 may be operated bytouching the display portion 7000 with a finger or the like.Furthermore, the remote controller 7111 may be provided with a displayportion for displaying information output from the remote controller7111. With operation keys or a touch panel of the remote controller7111, channels and volume can be controlled and images displayed on thedisplay portion 7000 can be controlled.

Note that the television device 7100 is provided with a receiver, amodem, and the like. With use of the receiver, general televisionbroadcasting can be received. When the television device is connected toa communication network with or without wires via the modem, one-way(from a transmitter to a receiver) or two-way (between a transmitter anda receiver or between receivers) information communication can beperformed.

FIG. 33(B) illustrates a laptop personal computer 7200. The laptoppersonal computer 7200 includes a housing 7211, a keyboard 7212, apointing device 7213, an external connection port 7214, and the like. Inthe housing 7211, the display portion 7000 is incorporated.

The display device of one embodiment of the present invention can beused in the display portion 7000. In this way, the laptop personalcomputer 7200 can display a high-resolution image. The laptop personalcomputer 7200 can display a high-resolution image on a large screen.

FIGS. 33(C) and (D) illustrate examples of the digital signage.

A digital signage 7300 illustrated in FIG. 33(C) includes a housing7301, the display portion 7000, a speaker 7303, and the like. Moreover,the digital signage 7300 can include an LED lamp, operation keys(including a power switch or an operation switch), a connectionterminal, a variety of sensors, a microphone, and the like.

FIG. 33(D) illustrates a digital signage 7400 attached to a cylindricalpillar 7401. The digital signage 7400 includes the display portion 7000provided along a curved surface of the pillar 7401.

The display device of one embodiment of the present invention can beused in each of the display portions 7000 illustrated in FIGS. 33(C) and(D). In this way, the digital signage 7300 and the digital signage 7400can display a high-resolution image. The digital signage 7300 and thedigital signage 7400 can display a high-resolution image on a largescreen.

A larger area of the display portion 7000 can provide more informationat a time. In addition, the larger display portion 7000 attracts moreattention, so that the effectiveness of the advertisement can beincreased, for example.

The use of a touch panel in the display portion 7000 is preferablebecause in addition to display of a still image or a moving image on thedisplay portion 7000, intuitive operation by a user is possible. In thecase where the display device is used for providing information such asroute information or traffic information, usability can be enhanced byintuitive operation.

Furthermore, as illustrated in FIGS. 33(C) and (D), it is preferablethat the digital signage 7300 or the digital signage 7400 work with aninformation terminal 7311 or an information terminal 7411 such as asmartphone a user has through wireless communication. For example,information of an advertisement displayed on the display portion 7000can be displayed on a screen of the information terminal 7311 or theinformation terminal 7411. Moreover, by operation of the informationterminal 7311 or the information terminal 7411, a displayed image on thedisplay portion 7000 can be switched.

Furthermore, it is possible to make the digital signage 7300 or thedigital signage 7400 execute a game with use of the screen of theinformation terminal 7311 or the information terminal 7411 as anoperation means (controller). Thus, an unspecified number of people canjoin in and enjoy the game concurrently.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Example 1

In this example, results of rough estimation of data writing time of an8K4K liquid crystal display module including a 65-inch diagonal pixelarea are described.

In particular, in this example, whether a large-sized high-resolutiondisplay in which hydrogenated amorphous silicon (a-Si:H) is used for asemiconductor layer of a transistor can be operated by applying oneembodiment of the present invention was examined.

Note that the resolution of the 8K4K display is extremely high: thehorizontal resolution is 7680 and the vertical resolution is 4320.Recommendation ITU-R BT.2020-2 is an international standard for 8K4Kdisplays. In the standard, the driving method is a progressive methodand the maximum frame frequency is 120 Hz.

In the case where a transistor with a low field-effect mobility is usedin a large-sized high-resolution display module, image rewriting cannotbe done in a frame period and driving cannot be performed in some cases.In such a case, a configuration in which a pixel area is divided into aplurality of parts (e.g., four parts) and each part is provided with ascan line driver circuit (also referred to as a gate driver) and asignal line driver circuit (also referred to as a source driver) can beemployed. With such a configuration, image rewriting of the plurality ofpixel areas can be performed at the same time; thus, image rewriting canbe performed in a frame period even when a transistor with a lowfield-effect mobility is used.

However, the configuration in which the pixel area is divided has, forexample, the following problems: an increase in cost resulting from anincrease in the number of ICs such as the source driver and the gatedriver and the amount of materials thereof; a decrease in the apertureratio due to an increase in the number of wirings; an increase in aframe area due to mounting of ICs; the necessity of a circuit forsynchronizing the divided pixel areas; and a decrease in visibilitybecause a boundary portion between the divided pixel areas is visuallyrecognized. In addition, image processing or the like for dividing imagedata to be input is necessary; thus, a large-scale image processingcircuit that can operate at a high speed might be required.

In view of the above, in this example, a configuration in which aselection signal was supplied to each gate line and pixels were selectedone by one and a configuration in which a selection signal was suppliedto two or four gate lines at a time and two or four pixels that adjoinedin the column direction were concurrently selected were considered. Thetwo or four pixels that were concurrently selected were connected todifferent source lines. That is, two or four source lines were arrangedfor each column. In this example, rough estimation of data writing timewas performed with use of a layout of pixels with such a configuration.

In this example, a case where hydrogenated amorphous silicon was usedfor a semiconductor layer of a transistor and a case where a metal oxidewas used for a semiconductor layer of a transistor were examined.

The data writing time in the case where hydrogenated amorphous siliconwas used for a semiconductor layer was estimated with use of a pseudoparameter obtained by changing field-effect mobility that is a designparameter from an actually measured value of a transistor formed usingmicrocrystalline silicon.

As to the semiconductor layer including a metal oxide, the following twotypes of structures were considered. As the metal oxide, In—Ga—Zn oxidewas used. In a first type, a single layer of a metal oxide with anatomic ratio In:Ga:Zn=1:1:1 or the neighborhood thereof was used as thesemiconductor layer. In a second type, a stacked layer of a metal oxidewith an atomic ratio In:Ga:Zn=4:2:3 or the neighborhood thereof was usedas the semiconductor layer. Specifically, a case where a CAC-OS(Cloud-Aligned Composite oxide semiconductor) film was used as a firstmetal oxide layer and a CAAC-OS (c-axis-aligned crystalline oxidesemiconductor) film was used as a second metal oxide layer was assumed.

Table 1 shows parameters of layers used in this example. Theseparameters were for the case of a transistor in which a metal oxide wasused for a semiconductor layer; however, in this example, the sameparameters were also used in the case of a transistor in whichhydrogenated amorphous silicon was used for a semiconductor layer.

TABLE 1 Relative Sheet dielectric Material Thickness resistance constantCounter electrode ITSO  100 nm   100 Ω/□ — Liquid crystal LC material3200 nm 0.011 fF/μm² 4 layer Pixel electrode ITSO  100 nm   100 Ω/□ —Planarization film acrylic 3000 nm 0.012 fF/μm² 4 Passivation film 2 SiN 100 nm 0.620 fF/μm² 7 Passivation film 1 SiON\SiON  430 nm 0.082 fF/μm²4 SD wiring* Cu  600 nm*** 0.050 Ω/□ — Semiconductor IGZO  40 nm — —layer or a-Si:H Gate insulating SiON  280 nm 0.127 fF/μm² 4 layer** Gatewiring* Cu  600 nm*** 0.050 Ω/□ — Substrate glass — — — *The conversionvalue based on sheet resistance 0.1 Ω/□ of TaN_10 nm\Cu_300 nm. **Theconversion value of an SiON single layer based on SiN_400 nm\SiON_50 nm.***700 nm in the case where IGZO was used for the semiconductor layerand two pixels were concurrently selected.<Case where Pixels are Selected One by One>

FIG. 34(A) is a block diagram showing a configuration of a displaymodule used in this example. In this configuration, a selection signalis supplied to each gate line and pixels are selected one by one. A gatedriver and a source driver are both external circuits. A gate line issupplied with the same signal from two gate driver ICs (External). Asource line is supplied with a signal from one source driver IC(External). A pixel area is not divided. The pixel area has a diagonalof 65 inches, and the number of effective pixels is 7680×RGB (H)×4320(V).

FIG. 34(B) is a circuit diagram of a pixel PIX(i, j). The pixel PIX(i,j) includes a transistor M1, a capacitor C1, and a liquid crystalelement LC. A gate of the transistor M1 is connected to a gate lineGL(i). One of a source and a drain of the transistor M1 is connected toa source line SL(j), and the other is connected to one electrode of thecapacitor C1 and one electrode of the liquid crystal element LC. Theother electrode of the capacitor C1 is connected to a wiring CSCOM. Theother electrode of the liquid crystal element LC is connected to awiring TCOM.

FIGS. 35(A) and (B) illustrate a pixel layout of a display module inwhich pixels are selected one by one. FIG. 35(A) is a top view in whicha stacked-layer structure including components from the gate line GL(i)to the pixel electrode is seen from the pixel electrode side. FIG. 35(B)is a top view in which the pixel electrode is omitted from FIG. 35(A).

The pixel size is 62.5 μm×187.5 μm. The transistor M1 is achannel-etched transistor with a bottom-gate top-contact structure. Thetransistor M1 has a channel length L of 4 μm, a channel width W of 8 μm,and an LDD region overlapping with a gate (hereinafter, an overlap LDDregion L_(ov)) of 2 μm. The gate line GL(i) has a width of 10 μm, andthe wiring CSCOM has a width of 3.5 μm. The source line SL(j) has awidth of 10 μm, but has a width of 4 μm at a portion crossing anotherwiring (the gate line GL(i) or the wiring CSCOM). The aperture ratio is45.6%.

First, rough estimation of data writing time in the case where a metaloxide is used for a semiconductor layer is described with reference toFIG. 36.

A period for charging a gate line of a pixel and a period for charging asource line and the pixel were roughly estimated in such a manner thatthe parasitic resistance and the parasitic capacitance were extractedfrom the pixel layout in FIG. 35(A) and only a parameter of thefield-effect mobility of the transistor was changed. In this example,the data writing time corresponds to the sum of the period for charginga gate line and the period for charging a source line and a pixel. Inthis example, the period for charging a gate line is a time until thepotential of the gate line reaches 75% of the maximum input voltage, andthe period for charging a source line and a pixel is a time until thepotential of the source line reaches 99% of the maximum input voltage.

Here, a normalized value (normalized mobility) under a condition thatthe field-effect mobility in the case where a stacked layer of a metaloxide with an atomic ratio In:Ga:Zn=4:2:3 or the neighborhood thereofwas used as a semiconductor layer was 1 was used. The transistor sizewas not changed. The load of the whole pixel area is described below. Aparasitic resistance Rgl of the gate line was 3.60 kΩ, a parasiticcapacitance Cgl of the gate line was 255 pF, a parasitic resistance Rslof the source line was 5.80 kΩ, a parasitic capacitance Csl of thesource line was 147 pF, and a parasitic capacitance Cpix of the pixelwas 216.6 fF. Note that in this example, the parasitic capacitance Cpixof the pixel includes storage capacitance of a capacitor, capacitance ofa liquid crystal element, and parasitic capacitance of a node A. In thisexample, the node A is a node at which a source or a drain of atransistor, one electrode of a capacitor, and one electrode of a liquidcrystal element are connected in each pixel.

The result of normalized mobility of 1 in FIG. 36 corresponds to thecase where a stacked layer of a metal oxide with an atomic ratioIn:Ga:Zn=4:2:3 or the neighborhood thereof is used as a semiconductorlayer (denoted as “CAC\CAAC” in FIG. 36). In that case, the data writingtime was 3.55 μs, which was shorter than one horizontal period 3.85 μsin 60-Hz driving;

accordingly, it was estimated that 60-Hz driving could be performed.This data writing time was longer than one horizontal period 1.93 μs in120-Hz driving; accordingly, it was estimated that 120-Hz driving wasdifficult.

The result of normalized mobility of 0.5 in FIG. 36 corresponds to thecase where a single layer of a metal oxide with an atomic ratioIn:Ga:Zn=1:1:1 or the neighborhood thereof is used as a semiconductorlayer (denoted as “IGZO(111)” in FIG. 36). In that case, the datawriting time was 4.17 μs, which was longer than one horizontal period3.85 μs in 60-Hz driving; accordingly, it was estimated that not only120-Hz driving but also 60-Hz driving was difficult.

Next, rough estimation of data writing time in the case wherehydrogenated amorphous silicon is used for a semiconductor layer isdescribed with reference to FIG. 37.

A period for charging a gate line of a pixel and a period for charging asource line and the pixel were roughly estimated in such a manner thatthe parasitic resistance and the parasitic capacitance were extractedfrom the pixel layout in FIG. 35(A) and the field-effect mobility of adesign parameter was changed from the actually measured value of atransistor fabricated using microcrystalline silicon. The transistorsize and storage capacitance were not changed. In the case wherehydrogenated amorphous silicon is actually used for a semiconductorlayer, a larger transistor and storage capacitance are needed, and thusthe data writing time needs to be longer than that shown as the resultin this example. The load of the whole pixel area is described below. Aparasitic resistance Rgl of the gate line was 3.60 kΩ, a parasiticcapacitance Cgl of the gate line was 255 pF, a parasitic resistance Rslof the source line was 5.80 kΩ, a parasitic capacitance Csl of thesource line was 147 pF, and a parasitic capacitance Cpix of the pixelwas 216.6 fF.

The results of field-effect mobility of 0.6, 0.7, and 0.8 [cm²/Vs] inFIG. 37 correspond to the case where hydrogenated amorphous silicon isused for a semiconductor layer. In that case, each data writing time was19.66 μs, 16.19 μs, or 13.81 μs, which were longer than one horizontalperiod 1.93 μs in 120-Hz driving and one horizontal period 3.85 μs in60-Hz driving; accordingly, it was estimated that not only 120-Hzdriving but also 60-Hz driving was difficult.

<Case Where Two Pixels are Selected at a Time>

FIG. 38(A) is a block diagram showing a configuration of a displaymodule used in this example. In this configuration, a selection signalis supplied to two gate lines at a time, and two pixels that adjoin inthe column direction are selected at a time. A gate driver and a sourcedriver are both external circuits. A gate line is supplied with the samesignal from two gate driver ICs. The gate line GL₀(i) is electricallyconnected to the gate line GL(i) and the gate line GL(i+1), and pixelsin two rows of the i-th row and the (i+1)-th row are driven at a time. Asignal is supplied to a source line from one source driver IC. A pixelarea is not divided. The pixel area has a diagonal of 65 inches, and thenumber of effective pixels is 7680×RGB (H)×4320 (V).

FIG. 38(B) is a circuit diagram showing the pixel PIX(i, j) and a pixelPIX(i+1, j).

First, a configuration of the pixel PIX(i, j) is described. The pixelPIX(i, j) includes the transistor M1, the capacitor C1, and the liquidcrystal element LC. The gate of the transistor M1 is connected to thegate line GL(i). One of the source and the drain of the transistor M1 isconnected to the source line SL₁(j), and the other is connected to oneelectrode of the capacitor C1 and one electrode of the liquid crystalelement LC. The other electrode of the capacitor C1 is connected to thewiring CSCOM. The other electrode of the liquid crystal element LC isconnected to the wiring TCOM.

Next, a configuration of the pixel PIX(i+1, j) is described. The pixelPIX(i+1, j) includes a transistor M2, a capacitor C2, and the liquidcrystal element LC. A gate of the transistor M2 is connected to the gateline GL(i+1). One of a source and a drain of the transistor M2 isconnected to the source line SL₂(j), and the other is connected to oneelectrode of the capacitor C2 and one electrode of the liquid crystalelement LC. The other electrode of the capacitor C2 is connected to thewiring CSCOM. The other electrode of the liquid crystal element LC isconnected to the wiring TCOM.

FIGS. 39(A) and (B) illustrate a pixel layout of a display module inwhich two pixels are selected at a time. FIG. 39(A) is a top view inwhich a stacked-layer structure including components from the gate lineGL(i) to the pixel electrode is seen from the pixel electrode side. FIG.39(B) is a top view in which the pixel electrode is omitted from FIG.39(A).

The pixel size is 62.5 μm×187.5 μm. The transistor M1 is achannel-etched transistor with a bottom-gate top-contact structure. Thetransistor M1 has a channel length L of 4 μm, a channel width W of 8 μm,and an overlap LDD region L_(ov) of 2 μm. The gate line GL(i) has awidth of 10 μm, and the wiring CSCOM has a width of 3.5 μm. Each of thesource line SL₁(j) and the source line SL₂(j) has a width of 10 μm, buthas a width of 4 μm at a portion crossing the gate line. The apertureratio is 37.3%.

First, rough estimation of data writing time in the case where a metaloxide is used for a semiconductor layer is described with reference toFIG. 40.

A period for charging a gate line of a pixel and a period for charging asource line and the pixel were roughly estimated in such a manner thatthe parasitic resistance and the parasitic capacitance were extractedfrom the pixel layout in FIG. 39(A) and only a parameter of thefield-effect mobility of the transistor was changed. Here, a normalizedvalue (normalized mobility) under a condition that the field-effectmobility in the case where a stacked layer of a metal oxide with anatomic ratio In:Ga:Zn=4:2:3 or the neighborhood thereof was used as asemiconductor layer was 1 was used. The transistor size was not changed.The load of the whole pixel area is described below. A parasiticresistance Rgl of the gate line was 3.60 kΩ, a parasitic capacitance Cglof the gate line was 364 pF, a parasitic resistance Rsl of the sourceline was 4.83 kΩ, a parasitic capacitance Csl of the source line was 182pF, and a parasitic capacitance Cpix of the pixel was 191 fF.

The result of normalized mobility of 1 in FIG. 40 corresponds to thecase where a stacked layer of a metal oxide with an atomic ratioIn:Ga:Zn=4:2:3 or the neighborhood thereof is used as a semiconductorlayer (denoted as “CAC\CAAC” in FIG. 40). In that case, the data writingtime was 3.49 μs, which was shorter than one horizontal period 3.83 μsin 120-Hz driving; accordingly, it was estimated that 120-Hz drivingcould be performed.

The result of normalized mobility of 0.5 in FIG. 40 corresponds to thecase where a single layer of a metal oxide with an atomic ratioIn:Ga:Zn=1:1:1 or the neighborhood thereof is used as a semiconductorlayer (denoted as “IGZO(111)” in FIG. 40). In that case, the datawriting time was 4.02 μs, which was shorter than one horizontal period7.66 μs in 60-Hz driving; accordingly, it was estimated that 60-Hzdriving could be performed. This data writing time was longer than onehorizontal period 3.83 μs in 120-Hz driving; accordingly, it wasestimated that 120-Hz driving was difficult.

In FIG. 40, two gate lines are supplied with the same selection signal,so that the length of one horizontal period can be twice the length ofone horizontal period in FIG. 36. Accordingly, a high-resolution displaydevice can be operated easily with use of a transistor with a lowfield-effect mobility.

The results in FIG. 36 and FIG. 40 show that in the case where CAC\CAACis used for a semiconductor layer, 120-Hz driving, which was difficultwith the configuration in which writing was performed in pixels one byone, can be achieved with the configuration in which writing isperformed in two pixels at a time.

The results in FIG. 36 and FIG. 40 show that in the case where IGZO(111)is used for a semiconductor layer, 60-Hz driving, which was difficultwith the configuration in which writing was performed in pixels one byone, can be achieved with the configuration in which writing isperformed in two pixels at a time.

Next, rough estimation of data writing time in the case wherehydrogenated amorphous silicon is used for a semiconductor layer isdescribed with reference to FIG. 41.

A period for charging a gate line of a pixel and a period for charging asource line and the pixel were roughly estimated in such a manner thatthe parasitic resistance and the parasitic capacitance were extractedfrom the pixel layout in FIG. 39(A) and the field-effect mobility of adesign parameter was changed from the actually measured value of atransistor fabricated using microcrystalline silicon. The transistorsize and storage capacitance were not changed. The load of the wholepixel area is described below. A parasitic resistance Rgl of the gateline was 3.60 kΩ, a parasitic capacitance Cgl of the gate line was 364pF, a parasitic resistance Rsl of the source line was 4.83 kΩ, aparasitic capacitance Csl of the source line was 182 pF, and a parasiticcapacitance Cpix of the pixel was 191 fF.

The results of field-effect mobility of 0.6, 0.7, and 0.8 [cm²/Vs] inFIG. 41 correspond to the case where hydrogenated amorphous silicon isused for a semiconductor layer. In that case, each data writing time was17.98 μs, 14.89 μs, or 12.78 μs, which were longer than one horizontalperiod 3.83 μs in 120-Hz driving and one horizontal period 7.66 μs in60-Hz driving; accordingly, it was estimated that not only 120-Hzdriving but also 60-Hz driving was difficult.

From the result in FIG. 41, it was estimated that in the case wherehydrogenated amorphous silicon was used for a semiconductor layer, 60-Hzdriving was difficult even with the configuration in which writing wasperformed in two pixels at a time, which was different from the casewhere a metal oxide was used for a semiconductor layer (see the resultin FIG. 40).

<Case Where Four Pixels are Selected at a Time>

The block diagram illustrating the structure of the display module usedin this example is the same as that illustrated in FIG. 1 except thatonly one source driver 13 is provided. The pixel area has a diagonal of65 inches, and the number of effective pixels is 7680×RGB (H)×4320 (V).In addition, the circuit diagram of the pixel provided in the pixel areais the same as that in FIG. 7, and the pixel layout is the same as thosein FIGS. 8(A) and (B).

The pixel size is 62.5 μm×187.5 μm. The transistors provided in thepixel are channel-etched transistors with a bottom-gate top-contactstructure and have the same size. Specifically, each of the transistorsprovided in the pixel has a channel length L of 4 μm, a channel width Wof 8 μm, and an overlap LDD region L_(ov) of 3 μm. Each gate line has awidth of 10 μm, and each wiring CS has a width of 5 μm. Each source linehas a width of 4 μm. The aperture ratio is 29%.

First, rough estimation of data writing time in the case where a metaloxide is used for a semiconductor layer is described with reference toFIG. 42.

A period for charging a gate line of a pixel and a period for charging asource line and the pixel were roughly estimated in such a manner thatthe parasitic resistance and the parasitic capacitance were extractedfrom the pixel layout in FIG. 8, and only a parameter of the mobilitywas changed. Here, a normalized value (normalized mobility) under acondition that the field-effect mobility in the case where a stackedlayer of a metal oxide with an atomic ratio In:Ga:Zn=4:2:3 or theneighborhood thereof was used as a semiconductor layer was 1 was used.The transistor size was not changed. The load of the whole pixel area isdescribed below. A parasitic resistance Rgl of the gate line was 3.53kΩ, a parasitic capacitance Cgl of the gate line was 518 pF, a parasiticresistance Rsl of the source line was 10.28 kΩ, a parasitic capacitanceCsl of the source line was 170 pF, and a parasitic capacitance Cpix ofthe pixel was 99.7 fF.

The result of normalized mobility of 1 in FIG. 42 corresponds to thecase where a stacked layer of a metal oxide with an atomic ratioIn:Ga:Zn=4:2:3 or the neighborhood thereof is used as a semiconductorlayer (denoted as “CAC\CAAC” in FIG. 42). In that case, the data writingtime was 5.05 μs, which was shorter than one horizontal period 7.61 μsin 120-Hz driving; accordingly, it was estimated that 120-Hz drivingcould be performed.

The result of normalized mobility of 0.5 in FIG. 42 corresponds to thecase where a single layer of a metal oxide with an atomic ratioIn:Ga:Zn=1:1:1 or the neighborhood thereof is used as a semiconductorlayer (denoted as “IGZO(111)” in FIG. 42). In that case, the datawriting time was 5.22 μs, which was shorter than one horizontal period7.61 μs in 120-Hz driving; accordingly, it was estimated that 120-Hzdriving could be performed.

In FIG. 42, four gate lines are supplied with the same selection signal,so that the length of one horizontal period can be four times the lengthof one horizontal period in FIG. 36. Accordingly, a high-resolutiondisplay device can be operated easily with use of a transistor with alow field-effect mobility.

The result in FIG. 42 shows that 120-Hz driving can be achieved with theconfiguration in which writing is performed in four pixels at a time,even when IGZO(111) whose mobility is lower than that of CAC\CAAC isused for a semiconductor layer.

Next, rough estimation of data writing time in the case wherehydrogenated amorphous silicon is used for a semiconductor layer isdescribed with reference to FIG. 43.

A period for charging a gate line of a pixel and a period for charging asource line and the pixel were roughly estimated in such a manner thatthe parasitic resistance and the parasitic capacitance were extractedfrom the pixel layout in FIG. 8 and the field-effect mobility of adesign parameter was changed from the actually measured value of atransistor fabricated using microcrystalline silicon. The transistorsize and storage capacitance were not changed. The load of the wholepixel area is described below. A parasitic resistance Rgl of the gateline was 3.53 kΩ, a parasitic capacitance Cgl of the gate line was 518pF, a parasitic resistance Rsl of the source line was 10.28 kΩ, aparasitic capacitance Csl of the source line was 170 pF, and a parasiticcapacitance Cpix of the pixel was 99.7 fF.

The results of field-effect mobility of 0.6, 0.7, and 0.8 [cm²/Vs] inFIG. 43 correspond to the case where hydrogenated amorphous silicon isused for a semiconductor layer. In that case, each data writing time was11.66 μs, 10.06 μs, or 9.01 μs, which was shorter than one horizontalperiod 15.3 μs in 60-Hz driving; accordingly, it was estimated that60-Hz driving could be performed. The data writing time was longer thanone horizontal period 7.61 μs in 120-Hz driving; accordingly, it wasestimated that 120-Hz driving was difficult.

The results in FIG. 37, FIG. 41, and FIG. 43 show that 60-Hz driving canbe achieved with the configuration in which writing is performed in fourpixels at a time, in the case where hydrogenated amorphous silicon isused for a semiconductor layer.

As described above, it was estimated that a large-sized high-resolutiondisplay, such as a display with a diagonal of 65 inches and a resolutionof 8K4K, could be operated by using one embodiment of the presentinvention, even when hydrogenated amorphous silicon is used for asemiconductor layer of a transistor.

REFERENCE NUMERALS

-   10 display device-   11 pixel-   12 a gate driver-   12 b gate driver-   13 source driver-   13 a source driver-   13 b source driver-   14 substrate-   15 substrate-   16 reference voltage generation circuit-   16 a reference voltage generation circuit-   16 b reference voltage generation circuit-   17 display portion-   18 a protective circuit-   18 b protective circuit-   19 a protective circuit-   19 b protective circuit-   20 liquid crystal element-   21 conductive layer-   22 liquid crystal-   23 conductive layer-   24 a alignment film-   24 b alignment film-   26 insulating layer-   30 transistor-   31 conductive layer-   31 a conductive layer-   32 semiconductor layer-   32 p semiconductor layer-   33 conductive layer-   33 a conductive layer-   33 b conductive layer-   33 c conductive layer-   34 insulating layer-   35 impurity semiconductor layer-   37 semiconductor layer-   38 opening portion-   39 a polarizing plate-   39 b polarizing plate-   41 coloring layer-   42 light-blocking layer-   50 light-   51 conductive layer-   52 conductive layer-   53 conductive layer-   54 conductive layer-   55 conductive layer-   60 capacitor-   71 opening portion-   72 opening portion-   73 opening portion-   74 opening portion-   81 insulating layer-   82 insulating layer-   84 insulating layer-   90 backlight unit-   121 a TAB tape-   121 b TAB tape-   131 a printed board-   131 b printed board-   132 a TAB tape-   132 b TAB tape-   200 a transistor-   200 b transistor-   200 c transistor-   200 d transistor-   200 e transistor-   200 f transistor-   211 insulating layer-   212 insulating layer-   212 a insulating layer-   212 b insulating layer-   212 c insulating layer-   212 d insulating layer-   215 insulating layer-   216 insulating layer-   216 a insulating layer-   221 conductive layer-   222 a conductive layer-   222 a_1 conductive layer-   222 a_2 conductive layer-   222 a_3 conductive layer-   222 b conductive layer-   222 b_1 conductive layer-   222 b_2 conductive layer-   222 b_3 conductive layer-   223 conductive layer-   224 insulating layer-   231 semiconductor layer-   231_1 semiconductor layer-   231_2 semiconductor layer-   231 d drain region-   231 i channel formation region-   231 s source region-   235 opening portion-   236 a opening portion-   236 b opening portion-   237 opening portion-   238 a opening portion-   238 b opening portion-   812 moving mechanism-   813 moving mechanism-   815 stage-   816 ball screw mechanism-   820 laser-   821 optical system unit-   822 mirror-   823 microlens array-   824 mask-   825 laser light-   826 laser light-   827 laser beam-   830 substrate-   840 amorphous silicon layer-   841 polycrystalline silicon layer-   7000 display portion-   7100 television device-   7101 housing-   7103 stand-   7111 remote controller-   7200 laptop personal computer-   7211 housing-   7212 keyboard-   7213 pointing device-   7214 external connection port-   7300 digital signage-   7301 housing-   7303 speaker-   7311 information terminal-   7400 digital signage-   7401 pillar-   7411 information terminal

1. A display device comprising: a first wiring, a second wiring, a thirdwiring, a transistor, a first conductive layer, a second conductivelayer, a third conductive layer, and a pixel electrode, wherein thefirst wiring extends in a first direction, wherein the second wiring andthe third wiring each extend in a second direction intersecting with thefirst direction, wherein a gate of the transistor is electricallyconnected to the first wiring, wherein one of a source and a drain ofthe transistor is electrically connected to the second wiring throughthe first conductive layer, the second conductive layer, and the thirdconductive layer, wherein the second conductive layer comprises a regionoverlapping with the third wiring, wherein the first conductive layer,the third conductive layer, and the pixel electrode comprise the samematerial, wherein the first wiring and the second conductive layercomprise the same material, wherein the first wiring is supplied with aselection signal, and wherein the second wiring and the third wiring aresupplied with different signals.
 2. The display device according toclaim 1, further comprising a first source driver and a second sourcedriver electrically connected to the second wiring and the third wiring.3.-5. (canceled)
 6. The display device according to claim 1, wherein thetransistor comprises a first semiconductor layer comprising amorphoussilicon.
 7. The display device according to claim 1, wherein thetransistor comprises a first semiconductor layer comprisingmicrocrystalline silicon or polycrystalline silicon.
 8. The displaydevice according to claim 1, wherein the transistor comprises a firstsemiconductor layer comprising a metal oxide.
 9. The display deviceaccording to claim 8, wherein the metal oxide contains indium, zinc, andM, and wherein the M is aluminum, titanium, gallium, germanium, yttrium,zirconium, lanthanum, cerium, tin, neodymium, or hafnium.
 10. (canceled)11. The display device according to claims 1, wherein the first wiringis a gate line, and wherein the second wiring and the third wiring are afirst source line and a second source line, respectively.
 12. A displaydevice comprising: a first wiring, a second wiring, a third wiring, atransistor, a first conductive layer, a second conductive layer, a thirdconductive layer, a pixel electrode, a first insulating layer, and asecond insulating layer, wherein the first wiring extends in a firstdirection, wherein the second wiring and the third wiring each extend ina second direction intersecting with the first direction, wherein a gateof the transistor is electrically connected to the first wiring, whereinone of a source and a drain of the transistor is electrically connectedto the second wiring through the first conductive layer, the secondconductive layer, and the third conductive layer, wherein the secondconductive layer comprises a region overlapping with the third wiring,wherein the first conductive layer, the third conductive layer, and thepixel electrode comprise the same material, wherein the first wiring andthe second conductive layer comprise the same material, wherein thefirst conductive layer is in contact with the one of the source and thedrain of the transistor through a first opening portion in the secondinsulating layer and in contact with the second conductive layer througha second opening portion in the first insulating layer and the secondinsulating layer, and wherein the third conductive layer is in contactwith the second conductive layer through a third opening portion in thefirst insulating layer and the second insulating layer and in contactwith the second wiring through a fourth opening portion of the secondinsulating layer.
 13. The display device according to claim 12, furthercomprising a first source driver and a second source driver electricallyconnected to the second wiring and the third wiring.
 14. The displaydevice according to claim 12, wherein the transistor comprises a firstsemiconductor layer comprising amorphous silicon.
 15. The display deviceaccording to claim 12, wherein the transistor comprises a firstsemiconductor layer comprising microcrystalline silicon orpolycrystalline silicon.
 16. The display device according to claim 12,wherein the transistor comprises a first semiconductor layer comprisinga metal oxide.
 17. The display device according to claim 16, wherein themetal oxide contains indium, zinc, and M, and wherein the M is aluminum,titanium, gallium, germanium, yttrium, zirconium, lanthanum, cerium,tin, neodymium, or hafnium.
 18. The display device according to claim12, wherein the first wiring is a gate line, and wherein the secondwiring and the third wiring are a first source line and a second sourceline, respectively.
 19. A manufacturing method of a display device, thedisplay device comprising: a first wiring, a second wiring, a thirdwiring, a transistor, a first conductive layer, a second conductivelayer, a third conductive layer, a pixel electrode, a first insulatinglayer, and a second insulating layer, wherein the first wiring extendsin a first direction, wherein the second wiring and the third wiringeach extend in a second direction intersecting with the first direction,wherein a gate of the transistor is electrically connected to the firstwiring, wherein one of a source and a drain of the transistor iselectrically connected to the second wiring through the first conductivelayer, the second conductive layer, and the third conductive layer,wherein the second conductive layer comprises a region overlapping withthe third wiring, wherein the first conductive layer, the thirdconductive layer, and the pixel electrode comprise the same material,and wherein the first wiring and the second conductive layer comprisethe same material, the method comprising the steps of: forming a firstopening portion and a fourth opening portion in the second insulatinglayer; forming a second opening portion and a third opening portion inthe first insulating layer and the second insulating layer; forming thefirst conductive layer in contact with the one of the source and thedrain of the transistor through the first opening portion and in contactwith the second conductive layer through the second opening portion; andforming the third conductive layer in contact with the second conductivelayer through the third opening portion and in contact with the secondwiring through the fourth opening portion.
 20. The manufacturing methodof a display device, according to claim 19, wherein the first wiring isa gate line, and wherein the second wiring and the third wiring are afirst source line and a second source line, respectively.